In the end, we all are who we are, no matter how much we may appear to have changed
So this is me taking the plunge and doing up some Willow meta. I think she has one of the best, most well-written character arcs in the show, and I'd kinda like to give props to it. I don't expect to convince anyone who doesn't like her to like her (you know who you are! *g*). Character preference isn't really connected to understanding. Hell, I still hate Angel no matter how many metas or fanfics I read about him (in fact, I usually hate him more after reading fanfic about him). So this isn't a "defense" of Willow. Just an explanation. :)
I'm quite fond of her character even though I don't talk about her a lot. I tend to post about Buffy more. See, I admire Buffy. However, I identify with Willow so talking about Willow is too much like talking about myself, and I'm never too happy to do that.
So this is probably the most Willow-y meta I'll ever do. :)
The main thing to keep in mind with Willow's character is that the one thing she prioritizes above all else is social harmony among her group. Willow can. not. stand it when one of her friends is upset or fighting or unhappy or when the dynamics are out of whack. It upsets her, and she wants to fix it.
That's not all there is going on with Willow, of course. But this main drive coupled with her own identity issues lead to the bitchiness and evilness of S6 Willow.
Now this meta isn't intending to explain every thing that Willow ever does. Cause...wow, that would take forever. But it is intending to give an overview of her arc and show how her character ticks so that every thing that Willow ever does can make more sense when you think about it. This meta also doesn't touch on Willow's magic-use very much. Her magic-use is one aspect of how her personality expresses herself and, well, it's the thing that gets discussed the most in fandom. However, there's more going on under the surface that, once you get a handle on it, makes the magic stuff make sense on its own. I'm going for the under-the-surface-y stuff here. (And, yes, you don't need to tell me Wrecked sucks. I know. That's not the point of this meta. So let's just ignore it, okay?)
I'm just gonna start at the beginning, though, because with Willow's character, you really have to go from the ground up.

Let's freeze Willow right before she meets Buffy in Welcome to the Hellmouth. Who is she?
Well, she's the nerdy girl whose mother picks out her clothes yet doesn't seem to pay much attention to her beyond that. She's ignored by her family and by much of the rest of the school. She's friends with Xander and Jesse, yet signs point to the boys relying on her mainly for study help. Willow identifies largely based on how other people see her, so she's concluded that there's little of interest about herself other than...schoolness.
This isn't too unusual. Cooley, a sociologist, came up with the looking-glass theory of self where a person builds up their self-image based on what's reflected back at them by the people around them. Willow is just a fabulous example of this, though, because of how her identity issues play out later. External affirmation is very important to Willow, and she develops her personality and identity on that basis.
So when we first meet her in S1, she's pretty much no one. She's a favorite target for Cordelia, and she is recognized as someone who will provide school help. And that's about it.
She's developed a crush on Xander, her life-long friend. This crush is safe for her. He's in her tiny social circle, and, perhaps more importantly, she's pretty sure it will never happen. This makes it extra-safe. She can play with the fantasy of a possible romance without actually risking anything.
Since she has so little going on with her socially, she doesn't really know how to be social. She hasn't had it modeled for her how to date or be friends or approach people. She doesn't get it. She mainly knows how to keep to herself and stay out of everyone's way.
From Welcome to the Hellmouth:
Buffy: Uh, Hi! Willow, right?
Willow: (looks up) Why? I-I mean, hi! Uh, did you want me to move?
Willow: (looks up) Why? I-I mean, hi! Uh, did you want me to move?
This is Willow pretty much as a walking mass of unfounded potential. Who she is, who she could be is not apparent. She doesn't even know.
The most important event of Willow's life is meeting Buffy. Buffy is probably the first person to treat Willow as an actual friend (in the girl way). She shows an actual interest in Willow, prompting Willow to reveal the parts of herself that she had thought nobody would care about. In S1, this means her crush on Xander, which is the main "thing" for Willow.
However, in S1, Willow's still largely a seemingly blank slate. She acts as a happy sounding board for Buffy, and she seems more interested in Buffy's love life than her own most of the time. This is because Willow still doesn't think she's very interesting. She feels more comfortable talking about other people rather than herself, so she happily listens to Buffy's woes and tribulations while providing something of a Yes-woman response.
Cause Willow doesn't want to do anything to drive away any of her friends right now. In S1, she becomes part of a cohesive group, maybe for the first time. Suddenly, she has a point of identification, and her interest is in keeping the group together. Without that, she loses what makes her interesting.
So Buffy's always right. Buffy/Angel is THE thing (because it's THE thing for Buffy, and Buffy's always right). Xander is a far-off daydream. And the pre-Scoobies are the center of the universe.

And so we move on to S2, where Willow becomes established even more.
There are three main events of importance to Willow's development in this season: Oz, Xander, and magic.
Let's start with Oz.
Oz is the first guy to show any interest in Willow. Well, besides Malcom of IRYJ (and we see how quickly Willow fell for him). There's nothing more attractive than someone else liking you. Remember that Willow's received very little external validation up till now. With Oz, she gets it in spades. He's head over heels over her. This is exciting and strange for Willow, but it's also a little scary.
In Phases, she worries about making the first move.
Buffy: Well, if you wanna up the speed quotient with Oz, maybe you need to do something daring. Maybe you need to make the first move.
Willow: Well, that won't make me a slut?
Buffy: I think your reputation will remain intact.
Willow: Well, that won't make me a slut?
Buffy: I think your reputation will remain intact.
While Willow is wanting to get her relationship with Oz off the ground, she's concerned with how making the first move will make her look. Again, Willow placing importance on how others perceive her.
However, once she receives assurance from Buffy that her reputation will remain intact, Willow enthusiastically goes for it. And her relationship with Oz gives her an extreme boost in confidence through the rest of the season (and the next). Oz adores her. Being adored means that she's adorable. Willow learns that she's adorable. Not only that, but she learns that she can be confident and pro-active when it comes to romance. These are firsts for Willow.
Let's side-step onto Xander now. It's perhaps more accurate to say Cordy and Xander.
Willow learns in Innocence that Xander, her safe fantasy crush, is having a thing with Cordelia. Cordelia had become a half-hearted member of the Scoobies, and Willow had grudgingly accepted her into the group (remember how important the group is to Willow?). However, seeing Xander kissing Cordelia smashes apart Willow's other main source of identification from S1. She's devastated, but she returns to help the team.
And it's for the team that she quickly gets over it. After all, she now has Oz interested in her, so she has something new with which to center herself around. And the group is ultimately what matters. There's, of course, the short lapse in Bewitched, Bothered, and Bewildered when she's put under a love spell that resurrects those feelings she had for him. But by the end of the season when she's coming out of her short coma in Becoming 2, she calls Oz's name. In S2, Oz gives her a far more substantial image of herself. She's not just "the girl who likes Xander". She's "the girl who's dating Oz". She replaces something that was ethereal and out-of-reach with something real and exciting. Finally, she's starting to build up an image of herself that's rooted in reality and that she can be proud of. As she proudly says several times: She's dating a musician.
Let's now move onto the magic, though. When Ms. Calendar dies, Willow takes over her classes. She also starts looking through Ms. Calendar's magic paraphenalia. From I Only Have Eyes For You:
Willow: (puts things away) Well, I had good lesson plans. Ms. Calendar had them on her computer.
Giles: Yes, yes, she was very, um... uh, dedicated.
Willow: And I found a bunch of files and Internet sites on paganism and magic and stuff. (smiles)
Giles: Oh?
Willow: Yes, it's really interesting.
Giles: Yes, yes, she was very, um... uh, dedicated.
Willow: And I found a bunch of files and Internet sites on paganism and magic and stuff. (smiles)
Giles: Oh?
Willow: Yes, it's really interesting.
Willow's smart. She has an ordered and logical mind. She also has a natural curiosity and penchant for systems (she is a computer geek). She takes the opportunity presented to her and starts to learn magic. Suddenly, she's found something that's incredibly relevant to the Slaying. She can do more than provide research help and moral support: she can do spells. The first spell she attempts is the exorcism in IOHEFY.
This is of immense importance to Willow. For one, it benefits the group, and it'll help Buffy. She wants to do whatever possible to help Buffy. Also, it gives her yet another source of identity. She's now the girl who does magic and dates a musician. This is so much more interesting than the no one she used to be.
The magic is the one thing that will remain constant, though. Her love interests will shift, but the magic stays the same. This is something that is for her, alone. This is why she's so enthusiastic about it. About showing others what she can do. About learning more and more and becoming better and better.
Ultimately, S2 is where the first roots form for Willow's later growth. However, for most of the season, she remains firmly at Buffy's side, always trying to support her or encourage her or defend her or defer to her or *enter obsequious action here*.

S3 brings even more changes. More magic, more romance, and her vampire self.
Let's start with how she is at the beginning, though. From Dead Man's Party:
Buffy: You wouldn't understand.
Willow: (considers) Well, maybe I don't need to understand. Maybe I . . . I just need you to talk to me.
Buffy: How could I talk to you when you were avoiding me?
Willow: This isn't easy, Buffy! I know you're going through stuff, but . . . so am I.
Buffy: I know that you were worried about me, but--
Willow: No! I don't just mean that. I mean, my life! You know? I, um . . . I'm having all sorts of--I'm dating, I'm having serious dating with a *werewolf*, a-and I'm studying witchcraft and killing vampires, and I didn't have anyone (starts sobbing) to talk to about all this scary life stuff. And you were my best friend.
Willow: (considers) Well, maybe I don't need to understand. Maybe I . . . I just need you to talk to me.
Buffy: How could I talk to you when you were avoiding me?
Willow: This isn't easy, Buffy! I know you're going through stuff, but . . . so am I.
Buffy: I know that you were worried about me, but--
Willow: No! I don't just mean that. I mean, my life! You know? I, um . . . I'm having all sorts of--I'm dating, I'm having serious dating with a *werewolf*, a-and I'm studying witchcraft and killing vampires, and I didn't have anyone (starts sobbing) to talk to about all this scary life stuff. And you were my best friend.
Buffy's departure at the end of S2 left Willow without one of her key sources of identity. Remember what I said about Buffy in S1? She provides Willow with a friend. This may seem minor, but to Willow, it's like opening a whole new world for her. Suddenly, Willow's left bereft in a sea of changes with nobody to help her through.
We see a shift in dynamic with this, though. Previously, the Buffy/Willow friendship had been centered around...Buffy. Buffy was the more interesting one. She was the Slayer. She was caught up in a huge, over-the-top romance. Willow would listen and encourage and support Buffy in anything that happened. But for her own part, Willow didn't have much to share. Until now. Now, she has dating issues and magic issues and Slaying issues. Instead of needing Buffy for the identity of "friend" that she provides her, Willow now needs Buffy as a friend, plain and simple.
Willow's starting to come into her own in establishing herself.
She'll feel the same abandonment in Bad Girls when she notices Buffy becoming closer to Faith than to her. Willow's friendship with Buffy is of vital importance to her, and anything that disrupts it needs to be fixed somehow.
However, we also see in S3 her having a brief affair with Xander. Not only is she being validated by one guy, she's having her former fantasy-guy falling for her. Talk about a super boost to the ol' ego.
This confidence and validation of her attractiveness is important to the formation of her own self-image at this point. Also important is her run-in with her vamp alter-ego in Doppelgangland. In that episode, she's confronted with an ideal of herself as predatory and confident and in charge. In this situation, she's able to evaluate how she comes across to other people. It's like being confronted with the reality of that looking-glass self concept I spoke of above.
Willow: (haughtily) She bothered me. She's so weak and accommodating. She's always letting people walk all over her, (turns to face her) and then she gets cranky with her friends for no reason. I just *couldn't* let her live.
At the end, she initially seems reluctant to take advantage of the opportunity to develop. However, her encounter with Percy makes her reconsider.
Buffy: You wanna go out tonight?
Willow: Strangely, I feel like staying at home... (sadly) and doing my homework... and flossing... and dying a virgin.
Buffy: (sagely) You know, you can O.D. on virtue.
Willow: Between me and my evil self, I've got double guilt coupons. I see now where the path of vice leads. I mean, she messed up everything she touched. I don't ever want to be like that.
Percy: (comes up to them, a bit nervous) Hey. Uh, hi.
Willow: Oh, hi. Listen, I didn't have a chance...
Percy: (interrupts) Okay, so I did the outline for the paper on Roosevelt. (hands it to her) It turns out there were two President Roosevelts, so I didn't know exactly which one to do, so I did both.
He hands her the other one. Both are nicely bound in folders. Buffy gives Willow a knowing smile. Willow just stares at him in stunned amazement.
Percy: (respectfully) Um, and I know they're kinda, kinda short, but I can flesh them out. Oh, and here's the bibliography. (hands it to her) Um, and I can retype that if you want. You just let me know what I did wrong, and I'll get on it.
Willow is speechless, amazed by his sudden change in attitude. He starts to go, but comes back to lay an apple on the folders in her lap. He leaves again, hopping athletically over a low wall.
Buffy: (innocently) You wanna go out tonight?
Willow: (hopefully) 9:00 sound good?
Willow: Strangely, I feel like staying at home... (sadly) and doing my homework... and flossing... and dying a virgin.
Buffy: (sagely) You know, you can O.D. on virtue.
Willow: Between me and my evil self, I've got double guilt coupons. I see now where the path of vice leads. I mean, she messed up everything she touched. I don't ever want to be like that.
Percy: (comes up to them, a bit nervous) Hey. Uh, hi.
Willow: Oh, hi. Listen, I didn't have a chance...
Percy: (interrupts) Okay, so I did the outline for the paper on Roosevelt. (hands it to her) It turns out there were two President Roosevelts, so I didn't know exactly which one to do, so I did both.
He hands her the other one. Both are nicely bound in folders. Buffy gives Willow a knowing smile. Willow just stares at him in stunned amazement.
Percy: (respectfully) Um, and I know they're kinda, kinda short, but I can flesh them out. Oh, and here's the bibliography. (hands it to her) Um, and I can retype that if you want. You just let me know what I did wrong, and I'll get on it.
Willow is speechless, amazed by his sudden change in attitude. He starts to go, but comes back to lay an apple on the folders in her lap. He leaves again, hopping athletically over a low wall.
Buffy: (innocently) You wanna go out tonight?
Willow: (hopefully) 9:00 sound good?
Willow discovers that the whole "confident aggressive" thing kinda worked. That she can get her own way if she's willing to use a bit more assertiveness.
There's another interesting part of Doppelgangland, though, and it has to do with magic. This season has seen Willow experimenting a lot with magic. She's developed her powers and takes great pleasure in trying spells. It's her "thing". When Anya initially approaches her about doing a spell, Willow is excited. However, when the spell goes awry and Willow sees the weird hell dimension, her reaction is strangely at odds with what we'll see from her later:
Willow: (reaches down, haughtily) I believe these chicken feet are mine. Look, m-magic is dangerous, Anya, i-it's, it's not to be toyed with. Now, if you'll excuse me, I have someone else's homework to do.
Again, we see Doppelgangland as the turning point in Willow's character this early on. The episode shows her how she could be and what she could accomplish. After this, we'll see her standing up to Faith in Choices and then casting spells with few reservations.
Ultimately, though, S3 sees Willow come into her own. She's no longer a lackey for Buffy. Her own "stuff" is of equal importance to her. Her stuff is definitely centered around the Slaying, though. So much that she gives up a better education to stay in Sunnydale. From Choices:
Willow: Actually, this isn't about you. Although I'm fond, don't get me wrong, of you. The other night, you know, being captured and all, facing off with Faith. Things just, kind of, got clear. I mean, you've been fighting evil here for three years, and I've helped some, and now we're supposed to decide what we want to do with our lives. And I just realized that that's what I want to do. Fight evil, help people. I mean, I-I think it's worth doing. And I don't think you do it because you have to. It's a good fight, Buffy, and I want in.
She's firmly established her identity around fighting evil. So much so that she can't imagine abandoning it. Of course, this makes Buffy and the rest of the Scoobies the core of her identity. She'll take great care to try to keep things stable. However, it's not all about Buffy anymore. Buffy's time in LA gave Willow space to establish herself separately from Buffy in a way she hadn't been able to before. Now, she's willing to disagree with Buffy, even though she ultimately remains supportive. Her role has developed from "hanger-on" to "fully-fledged friend", though. Willow now has a life independent of Buffy.
Having this positive self-image improves Willow's confidence to the point where we see her actually having friends outside the group (or at least friendly acquaintances). She's involved in school activities (through tutoring Percy). And she even thinks she'll miss high school. She's made an impact where she hadn't been previously.

Let's sidebar at this point to talk about social harmony within the context of the first three seasons.
I know it may sound a little odd to claim that Willow's primary goal is social harmony, especially when you get to the later seasons. It must be understood that what Willow considers to be ideal group dynamics aren't always as such objectively.
The Scoobies are likely Willow's first substantial social group. She's learning from the ground up. Most people learn to interact socially through their family, and Willow's family doesn't seem that connected with each other. She's unaccustomed to dealing with social situations, so she learns the mechanics of it in a rather intellectual fashion. Ironically, this makes her somewhat like Anya. However, unlike Anya, Willow is more successful at picking up the social graces because she has an intuitive gift for systems. Once she knows the ins and outs of what to say, when, to whom, she figures out how to play things to her advantage, whatever that advantage may be.
This advantage is almost always to keep the group stable. The only time this varies is when she's attempting to defend her place in the group (usually involving magic-use).
In the early seasons, Willow doesn't feel confident enough to actually do much in this regard. During the big Scooby fight in Becoming 1, the most Willow does is give Xander nasty looks and ask Buffy what she wants to do.
However, fast forward just half a season. In S3's Revelations when the Scoobies confront Buffy about Angel being back, Willow is much more outspoken and pro-active in...well...trying to get everyone to use "I" statements. She attempts (unsuccessfully) to play the role of mediator.
Willow: (trying to mediate) This isn't about attacking Buffy. Remember, 'I' statements only. 'I feel angry.' 'I feel worried.'
However, she's still not at the point of actively confronting other people. Back in Dead Man's Party, she didn't feel comfortable talking to Buffy about how Buffy's departure had affected her. It isn't until Buffy looks to be running away again that Willow is prompted to say something.
It's all about confidence at this point. Willow's still working on it.
In Consequences, we see Willow actually confronting Buffy with her grievance, though she does it in a very insecure way.
Willow: (stands up) I mean, don't get me wrong. I-I completely understand why you and Faith have been doing the bonding thing. You guys work together. You... You should get along.
Buffy: It's more complicated than that.
Willow: But, see, it's that exact thing that-that's just ticking me off! It's this whole 'Slayers only' attitude. I mean, since when wouldn't I understand? You, you talk to me about *everything*. I-it's like all of a sudden I-I'm not cool enough for you because I can't kill things with my bare hands.
Buffy suddenly bursts into tears. Willow is aghast at this result.
Willow: (regretfully) Oh! Oh, Buffy! Don't cry. (hugs her close) I'm sorry. I-I was too hard on you. (lets go) Sometimes I unleash. I-I don't know my own strength. I-i-it's bad. I-I-I'm bad. I'm a bad, bad, bad person. (looks for forgiveness)
Buffy: It's more complicated than that.
Willow: But, see, it's that exact thing that-that's just ticking me off! It's this whole 'Slayers only' attitude. I mean, since when wouldn't I understand? You, you talk to me about *everything*. I-it's like all of a sudden I-I'm not cool enough for you because I can't kill things with my bare hands.
Buffy suddenly bursts into tears. Willow is aghast at this result.
Willow: (regretfully) Oh! Oh, Buffy! Don't cry. (hugs her close) I'm sorry. I-I was too hard on you. (lets go) Sometimes I unleash. I-I don't know my own strength. I-i-it's bad. I-I-I'm bad. I'm a bad, bad, bad person. (looks for forgiveness)
Doppelgangland is really the turning point for Willow's confidence. It allows her to explore a new version of herself, a more assertive version. From that point on, we see her being more open, more confrontational, more willing to voice her concerns.

Before diving into S4, itself, let's pause and take a look at where Willow is as the season opens. With a firmly rooted self-image and a new, more welcoming environment for her, Willow is blossoming as she never did in high school. Not only that, but she knows it. She knows that she's reached a new stage of her life. She's transitioned.
Willow's mind works in systems, remember. She divides up her life into stages, and she's just now approaching the "young adult" phase. Graduating high school was a big part of that. However, having sex with Oz is another big part of that. That means she's a grown up now, and so she's eager to be grown up. This means encouraging Buffy, her bestest friend in the entire world, to also have sex.
After all, social harmony requires all members of the group to be satisfied. And a non-romantically entangled Buffy is a potentially unhappy Buffy. So Willow is all over trying to get Buffy into a romance of some sort.
Really, S4 is such a transitional season. Even from the beginning, it's established as being distinct from the previous seasons. Willow, especially. She has a new, trendy hair style. New fashion sense. She's fitting in like she never had before. This is Willow finally getting life. And what's more, life is working for her.
As she now sees herself in a new phase of life, she takes the opportunity to sort of reinvent herself. Not dramatically. This is basically the Willow that had been lurking underneath the lack of confidence in the earlier seasons. Now with three years worth of social support and validation, she's comfortable being who she is.
What all this brings, though, is a new questioning of Buffy. Willow's slowly developed into her own independent person, and she's not so quick to follow Buffy's lead all the time anymore. From Fear, Itself:
Buffy: Will, I'm telling you...
Willow: You're telling me? You're telling me?!
Buffy: I can't do my job if I have to worry about each of your safety.
Willow: It's not your decision!
Buffy: Got to disagree with you there.
Willow: Oh, of course you do.
Xander: Let's all take a breath. Buffy, maybe...
Willow: Being the Slayer doesn't automatically make you boss. You're as lost as the rest of us.
Oz: What are we talking about?
Willow: It's a simple incantation, a guiding spell for travelers when they become lost or disoriented.
Buffy: And how does it work?
Willow: It conjures an emissary from the beyond that lights the way.
Buffy: Conjuring. Will, let's be realistic here. Okay, your basic spells are usually only fifty-fifty.
Willow: Oh yeah? Well, - so is your face!
Willow walks off while Buffy tries to figure out what that meant.
Buffy: What? What does that mean?
Willow turns around.
Willow: I'm not your sidekick!
Willow: You're telling me? You're telling me?!
Buffy: I can't do my job if I have to worry about each of your safety.
Willow: It's not your decision!
Buffy: Got to disagree with you there.
Willow: Oh, of course you do.
Xander: Let's all take a breath. Buffy, maybe...
Willow: Being the Slayer doesn't automatically make you boss. You're as lost as the rest of us.
Oz: What are we talking about?
Willow: It's a simple incantation, a guiding spell for travelers when they become lost or disoriented.
Buffy: And how does it work?
Willow: It conjures an emissary from the beyond that lights the way.
Buffy: Conjuring. Will, let's be realistic here. Okay, your basic spells are usually only fifty-fifty.
Willow: Oh yeah? Well, - so is your face!
Willow walks off while Buffy tries to figure out what that meant.
Buffy: What? What does that mean?
Willow turns around.
Willow: I'm not your sidekick!
Oh, so many things going on in this scene. Willow's newfound confidence is leading her to question Buffy's automatic leadership. Especially because Buffy is questioning Willow's magic abilities.
Remember how important the magic is to Willow. It's her significant method of contribution to the group. It's what allows her to be helpful. Without it, she's just an ordinary girl who can hack like mad. Nice, sure. But not nearly as useful as magic can be when it comes to fighting evil. The magic makes Willow an almost indispensable member of the team later on. Being able to do magic is her security net for ensuring her continued membership in the Scooby gang, which forms the center of her entire world. Having her abilities called into question is tantamount to having her entire purpose challenged. She reacts harshly, social harmony going out the window. There's no point keeping the group together if she doesn't have a place in it.
Perhaps the most significant part of the season, though, is meeting Tara.
Willow's previous love interest, Oz, had fallen for her while she was still a shy, awkward nerdy-type. Tara, however, falls for the new, improved Willow. In fact, Tara almost resembles Willow as she used to be.
Willow's relationship with Tara is an affirmation of her new identity and an opening to reject her old one. As Willow looks back at her past, she can see who she is now and who she used to be, and she finds that she looks down at her old self. She's come so far, and is so happy in her new roles with her new confidence, she can't possibly understand how anybody could have liked that awkward, younger version of herself.
This isn't an conscious split or a fake persona (a la Spike) or anything. No, this is just someone who's developed naturally and looks back with an eye roll at who they were before. Willow's a late-bloomer, in adolescent terms. She didn't "grow up", so to speak, until much later than everybody else. As she finds how comfortable she is now, she comes to look back at her pre-blooming self with disdain.
Tara is the final piece of the Willow puzzle. Tara encourages Willow's magic use (at first). Tara provides validation for Willow's shiny new identity. Tara has no knowledge of Willow's old identity.
It is with all this that Tara is the final push for Willow to attain what she considers to be her full, complete self. S1-S3 provided the building blocks for Willow, and Tara is the last piece. Everything that happens from this point on is a result of the different aspects of Willow building on each other, and sometimes conflicting with each other, as well as conflicting with others.

Now seems a good time to sidebar into another important factor of Willow's personality. Truth is, she's kinda manipulative.
I don't mean that just in the overt, magic mind-wiping sense. As she learns how to handle things socially, she also becomes adept at redirection and obfuscation to avoid discussing or fighting about topics she'd rather not deal with. Specifically, her magic-use.
There are two important scenes to look at here. The first is from Something Blue.
Willow: I know. I-I've been off. I-I even tried to do a spell last night. To have my will done? I was hoping it would make me feel better. But it just went ka-blooey.
Giles: A spell? I don't think it's wise for you to be doing that alone right now. Your energy's too unfocused.
Willow: Well, that's not true. I said I was off, not incompetent.
Giles: I only meant that you're grieving, and it might be wise if you took a break from doing spells without supervision.
Willow: So I get punished ‘cause I'm in pain?
Giles: It's not punishment. I'm only saying this because I—
Willow: Oh, you care. Yeah. Everybody cares. Nobody wants to be inconvenienced. You all want me to take the time and go through the pain, as long as you don't have to hear about it anymore.
Giles: No, that's not fair.
Willow: Isn't it? ‘Cause I'm doing the best I can and it doesn't seem to be enough for you guys.
Giles: And I see how you could feel that way, I do—
Willow: No, you don't. You say that you do, but you don't see anything.
Giles: A spell? I don't think it's wise for you to be doing that alone right now. Your energy's too unfocused.
Willow: Well, that's not true. I said I was off, not incompetent.
Giles: I only meant that you're grieving, and it might be wise if you took a break from doing spells without supervision.
Willow: So I get punished ‘cause I'm in pain?
Giles: It's not punishment. I'm only saying this because I—
Willow: Oh, you care. Yeah. Everybody cares. Nobody wants to be inconvenienced. You all want me to take the time and go through the pain, as long as you don't have to hear about it anymore.
Giles: No, that's not fair.
Willow: Isn't it? ‘Cause I'm doing the best I can and it doesn't seem to be enough for you guys.
Giles: And I see how you could feel that way, I do—
Willow: No, you don't. You say that you do, but you don't see anything.
This is such an interesting exchange. Look at what's going on. Willow's been acting "off" since Oz left her, and Giles shows concern. She mentions that she tried to do a spell that would make her feel better. Giles follows up by letting her know that doing magic while upset is not the best idea. However, Willow takes this as an insult, as him saying she's "incompetent". When Giles attempts to get the discussion back on the rails, Willow keeps twisting it back into something it isn't so she can avoid the topic altogether. Suddenly, being told to be careful doing magic while in a distraught magical state constitutes a "punishment". Then it becomes about Giles dismissing her pain wholesale and wanting her to get over it.
By the end, Giles hasn't made any progress in expressing his reservations about the magic Willow's been doing.
The next scene is in Tough Love.
Willow: S-O-R-T of. (Tara frowns) I mean, I just feel like the-the junior partner. You've been doing everything longer than me. You've been out longer ... you've been practicing witchcraft way longer.
Tara: Oh, but you're way beyond me there! In just a few- I mean ... it frightens me how powerful you're getting.
Willow: (frowns) That's a weird word.
Tara: (nervous smile) "Getting"?
Willow: It frightens you? *I* frighten you?
Tara:: (jumps up from the bed) That is *so* not what I meant. I meant i-impresses - impressive.
Willow: Well, I took Psych 101. I mean, I took it from an evil government scientist who was skewered by her Frankenstein-like creation before the final, but I know what a Freudian slip is.
Tara looks upset.
Willow: D-don't you trust me?
Tara: With my life.
Willow: That's not what I mean.
Tara: Can't we just go to the fair?
Willow: I don't feel real multicultural right now. (stands up) Wh ... what is it about me that you don't trust?
Tara: It's not that. I worry, sometimes. You're, you're changing so much, so fast. I don't know where you're heading.
Willow: Where I'm heading?
Tara: I'm saying everything wrong.
Willow: No, I think you're being pretty clear. This isn't about the witchcraft. It's about the other changes in my life.
Tara: I trust you. I just ... (looks down) I don't know where I'm gonna fit in ... in your life when...
Willow: When ... I change back? Yeah, this is a college thing, just a, a little experimentation before I get over the thrill and head back to boys' town.
Pause.
Willow: You think that?
Tara: Should I?
Willow: I'm really sorry that I didn't establish my lesbo street cred before I got into this relationship. You're the only woman I've ever fallen in love with, so ... how on earth could you ever take me seriously?
Tara: Oh, but you're way beyond me there! In just a few- I mean ... it frightens me how powerful you're getting.
Willow: (frowns) That's a weird word.
Tara: (nervous smile) "Getting"?
Willow: It frightens you? *I* frighten you?
Tara:: (jumps up from the bed) That is *so* not what I meant. I meant i-impresses - impressive.
Willow: Well, I took Psych 101. I mean, I took it from an evil government scientist who was skewered by her Frankenstein-like creation before the final, but I know what a Freudian slip is.
Tara looks upset.
Willow: D-don't you trust me?
Tara: With my life.
Willow: That's not what I mean.
Tara: Can't we just go to the fair?
Willow: I don't feel real multicultural right now. (stands up) Wh ... what is it about me that you don't trust?
Tara: It's not that. I worry, sometimes. You're, you're changing so much, so fast. I don't know where you're heading.
Willow: Where I'm heading?
Tara: I'm saying everything wrong.
Willow: No, I think you're being pretty clear. This isn't about the witchcraft. It's about the other changes in my life.
Tara: I trust you. I just ... (looks down) I don't know where I'm gonna fit in ... in your life when...
Willow: When ... I change back? Yeah, this is a college thing, just a, a little experimentation before I get over the thrill and head back to boys' town.
Pause.
Willow: You think that?
Tara: Should I?
Willow: I'm really sorry that I didn't establish my lesbo street cred before I got into this relationship. You're the only woman I've ever fallen in love with, so ... how on earth could you ever take me seriously?
Again, we have someone attempting to discuss their reservations about Willow's magic-use, and Willow handily side-stepping and diverting the argument into something else entirely. At this point, Tara lets her do this, and they end up fighting about the lesbian thing.
Is Willow doing this consciously and with malice? Most likely not. It's an avoidance strategy that she's probably not even aware of. She just knows that talking about how she handles magic is not something she's eager to do, especially with a mentor, such as Giles, or a girlfriend, such as Tara. When she's confronted with such an argument, she manages to find a way out of it. Later in S6, her methods will escalate from harshly cutting the other person off (such as in All the Way when Willow and Tara fight at the Bronze) to outright mind-wiping (also in All the Way).

Restless deals in depth in Willow's identity split, and it also deals with Willow's fear that she will be "found out". Not only does she show disdain for her old personality, she feels fear that she's still that girl. The girl who didn't have any social skills or friends or anything useful to contribute to anything. That scares her. Especially as she fears Tara would reject her if she found out.
From Two to Go:
Willow: (scoffs) Let me tell you something about Willow. (advancing toward Buffy) She's a loser. And she always has been. People picked on Willow in junior high school, high school, up until college. With her stupid mousy ways. And now? Willow's a junkie. [...] The only thing Willow was ever good for...the only thing I had going for me ... were the moments - just moments - when Tara would look at me and I was wonderful. (grimly) And that will never happen again.
Dark Willow is the natural culmination of pretty much all Willow's issues wrapped up in a tidy, apocalyptic package. The magic, the rejection of her old self, her attachment to Tara, her jury-rigged understanding of social behavior, her ardent desire to fix things. When she feels the pain of the world, she wants to end it. Much like her attempt at mind-wiping Tara and Buffy in Tabula Rasa, she thinks this is a good solution. It's a completely fucked up action, of course, but the motivation behind it is not-quite-malicious.
There is one final tipping point for Willow to become Dark Willow, though, and that's Buffy's death at the end of S5.
Remember how important Buffy is to Willow's self-image. Even now, when Willow feels she's fully self-actualized and independent, Buffy is the center that holds the group together. Without her, the glue that holds Willow's identity together begins to break apart. Her desperation to bring Buffy back leads her to darker and darker magic. She's willing to make more compromises and push things farther than she would have before.
However, there's another factor to this all in that Buffy's death allows Willow to take a spin as the leader.
From Bargaining:
Xander: It's just ... (fidgets nervously) It feels wrong.
Tara: It is wrong. (Willow looks surprised) It's against all the laws of nature, and practically impossible to do, but it's what we agreed to. If-if you guys are changing your minds-
Willow: Nobody's changing their minds. Period.
Xander: Excuse me? Who made you the boss of the group?
Anya: You did.
Tara: You said Willow should be boss. (Willow moves away)
Anya: And then you said "let's vote," and it was unanimous...
Tara: ...and then you made her this little plaque, that said "Boss of Us," you put little sparkles on it...
Xander: Valid points, all.
Tara: It is wrong. (Willow looks surprised) It's against all the laws of nature, and practically impossible to do, but it's what we agreed to. If-if you guys are changing your minds-
Willow: Nobody's changing their minds. Period.
Xander: Excuse me? Who made you the boss of the group?
Anya: You did.
Tara: You said Willow should be boss. (Willow moves away)
Anya: And then you said "let's vote," and it was unanimous...
Tara: ...and then you made her this little plaque, that said "Boss of Us," you put little sparkles on it...
Xander: Valid points, all.
Not only is Willow the leader, she's the unanimously-chosen leader of the group. This means that for the entire summer, she's been given the leeway and freedom to do things as she thinks they should be done. Instead of always deferring to Buffy, Willow's in charge. You remember by the end of S5 how Willow was questioning Buffy's decisions?
From Tough Love:
Buffy: You cannot even think about taking on Glory.
Willow: You saw what she did to Tara. I can't let her get away with it.
Buffy: No. You *have* to let her get away with it. Even I'm no match for her, you know that.
Willow: But maybe I am.
She turns to go but Buffy grabs her arm.
Buffy: You're not. And I won't let you go.
Willow: This is not your choice. It's mine.
Buffy: This is not the time.
Willow: When, Buffy? When is? When *you* feel like it? When it's someone *you* love as much as I love Tara? When it's Dawn, is that it?
Buffy: When we have a chance. We'll fight her, when we have a chance. You wouldn't last five minutes with her, Willow. She's a god.
Willow: (shakes her head sadly) Fine. I'll wait.
Buffy: It's the only way.
Willow: (skeptical) Yeah.
Willow: You saw what she did to Tara. I can't let her get away with it.
Buffy: No. You *have* to let her get away with it. Even I'm no match for her, you know that.
Willow: But maybe I am.
She turns to go but Buffy grabs her arm.
Buffy: You're not. And I won't let you go.
Willow: This is not your choice. It's mine.
Buffy: This is not the time.
Willow: When, Buffy? When is? When *you* feel like it? When it's someone *you* love as much as I love Tara? When it's Dawn, is that it?
Buffy: When we have a chance. We'll fight her, when we have a chance. You wouldn't last five minutes with her, Willow. She's a god.
Willow: (shakes her head sadly) Fine. I'll wait.
Buffy: It's the only way.
Willow: (skeptical) Yeah.
This example is just the latest in Willow's questioning of Buffy's decisions. This started way back in Fear, Itself. Buffy's death gives Willow the unchallenged freedom to make those decisions that Buffy would have been against.
This is not something easily given up when Buffy comes back. And in S6, Buffy's mental state allows Willow to continue holding the reins of the group. It's when Buffy starts coming out of her funk that she takes charge again, but by that point, Willow's demoralized by the whole "cold turkey no magic" thing, and she doesn't have the will to challenge Buffy...until Tara's death tips her over the edge.
Dark Willow is like over-saturated Willow. Willow to the max power. She's what Willow develops into when completely unrestrained by anything. Full access to magical power, full freedom to do whatever the fuck she wants to do, and without Tara, she has no reason to hold herself back. Tara acts as an anchor for Willow. When she dies, all bets are off.
But Dark Willow gets talked about an awful lot, and this meta isn't intended to discuss that part of Willow in great detail.

Okay, so you may be wondering why I don't talk about S7 or the gay thing or X or Y or Z. There's a good reason for that, and that's that this meta isn't about Willow's actions. It's about why Willow's doing those actions. And what I've laid out above gives the framework for just about everything Willow ever does. S7, the gay thing, X, Y, Z, and F can all be extrapolated from it.
Willow's story plays out so strangely on screen. Since Buffy is the primary viewpoint character, we don't often get why Willow's doing what she does. Especially in the early seasons when she's such a Buffy-sycophant, it can be hard to figure out what she's about. It's only when she blossoms in S3 and beyond that we start to see who was hiding beneath the shy exterior. Dark Willow becomes something of a representation of the turmoil that Willow's been going through as all the different forces of her personality clash and burst out.
Willow goes through such a significant change throughout the course of the series, and yet it feels so natural. In some ways, she's the flipside of Buffy in relation to the Slayer. Slaying isolated Buffy and tried to force her to be someone she wasn't (cold, hard, distant). On the other hand, the Slaying gave Willow friends and a place in life, and it allowed her to become who she truly is. Of course, it also allowed her to go too far and go off the rails, but hey, that's what makes good TV.
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Comments
Anyway, interesting, insightful meta. I don't have much to add (I suck at thinking about these things), but keep writing! :D
Glad you enjoyed. :)