"I've got a theory"
Buffy the show is about Buffy the character. What it means to be a superhero. How being "The one girl in all the world with the strength and skill to hunt and kill vampires, demons and the forces of darkness" affects you.
It's clear from the very beginning that Buffy doesn't want to be that girl:
Welcome to the Hellmouth
Giles: (interrupting) I know what you're after!
With a big grin on his face he pulls out a large old book with the word "VAMPYR" written in gold leaf on the front cover. Buffy looks up at him with an uneasy gaze.
Buffy: That's not what I'm looking for.
Giles: Are you sure?
Buffy: I'm *way* sure.
And this theme continues throughout the show:
Boy: You're just a girl
Buffy: That's what I keep saying
From season 1 to season 5 Buffy goes from being a fairly happy-go-lucky girl to more like a normal slayer, focused on the job, like we see her in The Wish, to someone totally overwhemled in The Weight of the World.
Buffy: I want to leave
Wesley: What? Now?
Buffy: No, not now. After I graduate, you know, college?
Wesley: But you're a slayer.
Buffy: Yeah, I'm also a person. You can't just define me by my slayerness. That's something-ism.
In the first two thirds of season 5 Buffy goes from being an only daughter to the mother figure to her sister, a very quick lesson in "Growing up".
She's destined to fight until she's dead, she can't quit her job, if she gives up everyone will die:
Buffy: So that's it, huh? I remember the drill. One Slayer dies, next one's called! Wonder who she is. (to Giles) Will you train her? Or will they send someone else?
Giles: Buffy, I...
Buffy: They say how he's gonna kill me? Do you think it'll hurt?
Tears are flowing freely from her eyes. Angel tries to hug her, but she puts up her hands and quickly steps away.
Buffy: Don't touch me! (to Giles) Were you even gonna tell me?
Giles: I was hoping that I wouldn't have to. That there was... some way around it. I...
Buffy: I've got a way around it. I quit!
Angel: It's not that simple.
Buffy: I'm making it that simple! I quit! I resign, I-I'm fired, you can find someone else to stop the Master from taking over!
Giles: I'm not sure that anyone else can. All the... the signs indicate...
Buffy: The signs? (throws a book at him) READ ME THE SIGNS! (throws another one) TELL ME MY FORTUNE! YOU'RE SO USEFUL SITTING HERE WITH ALL YOUR BOOKS! YOU'RE REALLY A LOTTA HELP!
Giles: No, I don't suppose I am.
Angel: I know this is hard.
Buffy: What do you know about this? You're never gonna die!
Angel: You think I want anything to happen to you? Do you think I could stand it? We just gotta figure out a way...
Buffy: I already did. I quit, remember? Pay attention!
Giles: Buffy, if the Master rises...
Buffy: (yanks the cross from her neck) I don't care! (calms down) I don't care. Giles, I'm sixteen years old. I don't wanna die.
Then after finding out about the kids who've been killed in the school:
Giles: Buffy's not gonna face the Master. I am.
Buffy: No, you're not.
Giles and Ms. Calendar look over and see her approaching.
Buffy: So, I'm looking for a kid, huh? And he'll lead me to the Master?
Giles: Buffy, I'm not gonna send you out there to die. Now, you were right. I-I've waded around in these old books for so long, I've forgotten what the real world is like. I-it's time I found out.
Buffy: You're still not going up against the Master.
Giles: I've made up my mind.
Buffy: So have I.
Giles: I made up my mine first! I'm older and wiser than you, and just... just do what you're told for once! Alright?
Buffy: That's not how it goes. I'm the Slayer.
Giles: I don't care what the books say. I defy prophecy, and I am going. There's nothing you can say will change my mind.
Buffy: I know.
She pretends to turn to go, but then throws a hard punch to his face, knocking him out. He falls backward to the floor.
Death is all around her, her job is to cause death, and she comes to accept it.
Fool for Love
Spike: Every day you wake up, it's the same bloody question that haunts you: is today the day I die? Death is on your heels, baby, and sooner or later it's gonna catch you. And part of you wants it... not only to stop the fear and uncertainty, but because you're just a little bit in love with it. Death is your art. You make it with your hands, day after day. That final gasp. That look of peace. Part of you is desperate to know: What's it like? Where does it lead you? And now you see, that's the secret. Not the punch you didn't throw or the kicks you didn't land. Every Slayer... has a death wish. Even you.
The most crushing blow for Buffy is the death of Joyce in The Body, which was compounded by the fact that her father wasn't in contact.
The Weight of the World
Buffy #1: (not turning) This was when I quit, Will.
Shot of Willow standing beside Buffy #2, both staring at Buffy #1.
Willow: You did?
Buffy #2: Just for a second.
Buffy #1: (facing Willow) I was in the magic shop.
Buffy #2: I put a book back for Giles.
Buffy #1: Nothing special about it. And then it hit me.
Willow: What hit you?
Buffy #2: I can't beat Glory.
Buffy #1: Glory's going to win.
Willow: (turns to Buffy #2) You can't know that.
Buffy #2: (turns to Willow) I didn't just know it.
Buffy #1: (staring at nothing) I felt it. Glory will beat me.
Buffy #2: (looks away) And in that second of knowing it, Will...
Buffy #1: I wanted it to happen.
Buffy #1: I wanted it over. This is ... all of this ... it's too much for me.
Buffy #2: (staring at nothing) I just wanted it over.
Buffy #1: If Glory wins ... then Dawn dies.
Buffy #2: And I would grieve. People would feel sorry for me. (looks at Willow) But it would be over. (looks away) And I imagined what a relief it would be.
Buffy #2: I killed Dawn.
Willow frowns, looks at Buffy #2.
Willow: Is that what you think?
Shot of the "real" Buffy sitting blankly in the bedroom.
Buffy voiceover: My thinking it made it happen.
Cut back to the magic shop.
Buffy #1: Some part of me wanted it. And in the moment Glory took Dawn...
Buffy #2: I know I could have done something better. But I didn't. I was off by some fraction of a second.
Buffy #1: And this is why...
Buffy #2: ...I killed my sister.
Willow: You've carried the weight of the world on your shoulders since high school. And I, I know you didn't ask for this, but ... you do it every day. And so, you wanted out for one second. So what?
Buffy #2: (pensive) I got Dawn killed.
Willow: Hello! Your sister, not dead yet! But she will be if you stay locked inside here and never come back to us.
Buffy #2: (looking at Buffy #1) But what if I can't?
Willow: Then I guess you're right. And you did kill your sister.
Buffy comes out of her coma
Giles: Um ... well, uh... according to these scrolls, uh, it's possible for Glory to be stopped.
Giles: Buffy, the only way is to kill Dawn.
Buffy refuses to accept this and finds an alternative solution, to kill herself.
To commit suicide is the perfect solution to Buffy: she saves the world, she saves her sister, and she frees herself from her job (as shown in the 99 episodes in the Previously on Buffy clip, everything leading up to this point), being the Slayer.
Buffy: I loved him so much. But I knew ... what was right. I don't have that any more. I don't understand. I don't know how to live in this world if these are the choices. If everything just gets stripped away. I don't see the point.
Buffy: Dawn, the hardest thing in this world ... is to live in it.
Of course we find out in Bargaining that she failed, as she's "rescued" by her friends. She even tries to repeat her actions and ascends the tower again.
In Afterlife she explains to Spike:
Buffy: I was happy. Wherever I ... was ... I was happy. At peace. I knew that everyone I cared about was all right. I knew it. Time ... didn't mean anything ... nothing had form ... but I was still me, you know? (glances at him, then away) And I was warm ... and I was loved ... and I was finished. Complete. I don't understand about theology or dimensions, or ... any of it, really ... but I think I was in heaven And now I'm not. (almost tearful) I was torn out of there. Pulled out ... by my friends. Everything here is ... hard, and bright, and violent. Everything I feel, everything I touch ... this is Hell. Just getting through the next moment, and the one after that ... knowing what I've lost...
She's looking for a reason to live, a reason to feel alive, and she attempt to find it with Spike.
Throuhout season 6 we see Buffy just staring at things, a running tap, a meat slicer, as though she's not interested in the world.
In Normal Again Buffy gets the chance to create a world in which she's not the Chosen One
Even before the demon ... I've been so detached.
Every day I try to ... snap out of it. Figure out why I'm like that.
Spike: I hope you don't think this antidote's gonna rid you of that nasty martyrdom. See, I figured it out, luv. You can't help yourself. You're not drawn to the dark like I thought. You're addicted to the misery. It's why you won't tell your pals about us. Might actually have to be happy if you did. They'd either understand and help you, god forbid ... or drive you out ... where you can finally be at peace, in the dark. With me. Either way, you'd be better off for it, but you're too twisted for that. Let yourself live, already. And stop with the bloody hero trip for a sec. We'd all be the better for it.
Buffy: I just wish my mom was here.
In Normal Again her mother was there:
JOYCE: Buffy? Buffy! Buffy, fight it. You're too good to give in, you can beat this thing. Be strong, baby, ok? (Buffy crying) I know you're afraid. I know the world feels like a hard place sometimes, but you've got people who love you. (tearful) Your dad and I, we have all the faith in the world in you. We'll always be with you.
CrazyBuffy calms down as these words start to sink in.
JOYCE: You've got ... a world of strength in your heart. I know you do. You just have to find it again. Believe in yourself.
Joyce strokes her hair. CrazyBuffy sniffles, looks determined. Slowly she turns her head to look Joyce in the eye.
BUFFY: You're right. Thank you.
Buffy does then what she didn't do in Bargaining, she chooses to return to the world of Sunnydale, to be the Slayer, to come back to life. And in Grave she tells Dawn:
Things have really sucked lately, but it's all gonna change. And I wanna be there when it does. I want to see my friends happy again. And I want to see you grow up... I got it so wrong. I don't want to protect you from the world. I want to show it to you.
In season 7 she's accepted her place in the world, and will live happily ever after. Unless the writers have anything to do with it :-)
Update: Season 7: nice ending, brings the whole story to a conclusion that resolves the problems brought up during the first episode and ever since, Buffy not wanting to be the chosen one.