Screencap and script quotations courtesy of Buffyworld.com
The last few weeks I've been reading a lot of LJ metas about BtVS and playing "catch up" on some fascinating conversations and insights. One of the most talked-about points in fandom is the meaning and motivation behind Buffy's decision to jump from the tower in Dawn's place in The Gift, and it generally comes down to the notion that it was both a heroic sacrifice AND a symbolic suicide, the Slayer death wish that Spike spoke of in Fool For Love writ large.
[My theory after the cut....]This seems to be reinforced by what Buffy says to Giles at the Magic Box before they go to rescue Dawn from Glory (from the shooting script)
I sacrificed Angel to save the world. 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's just stripped away then I don't see the point.
But her monologue ends with this:
I just wish... I wish my mom was here.
Joyce's death earlier in the season was of course a turning point for Buffy, and therefore the entire series: the death of her beloved mother, a genuine, mundane, irreversible death that no Slayer powers could have prevented. The event would have left Buffy bereft in any case, but it also leaves her entirely responsible for her sister Dawn, and informs all of her choices for the remainder of the season.
I think this point is overlooked in the discussions about Buffy's sacrifice, or at least I haven't seen it explicitedly stated in the discussions I've read. (Granted that I'm new to Buffy fandom and make no claim of having made an exhaustive search.) The theme of "family" in season 5 is certainly well-recognized, but I've noticed that after Joyce's death she disappears from the conversations by fans as discussion focuses on Buffy and Dawn for the obvious reasons that that's the focus of the story. But Buffy's mention of her mother in her speech to Giles is a reminder, IMO, that Joyce is "gone but not forgotten".
How this plays into Buffy's motivation in The Gift, I believe, can be explicitly tied back to the episode "Listening to Fear" (5.09) and it's final scene, pictured above: Joyce confirms with Buffy the knowlege she received, a sort of a vision, that Dawn was not really her daughter, which Buffy reluctantly confirms. Joyce then asks Buffy to promise to protect Dawn: (from shooting script)
And she's important. To the world. Precious...Promise me, Buffy - if anything happens to me, if I don't come through this...No matter what she is, she still feels like my daughter, and I have to know you'll keep her safe. You'll love her like I love you.
Joyce's death does not occur for another seven episodes (technically, six) but this scene serves the function of "Joyce's dying wish", the deathbed conversation that she and Buffy never have otherwise; Buffy finds her mother's body unexpectedly at the end of I Was Made to Love You (5.15).
Throughout the series Buffy has proven herself to be someone who takes on responsibility, or has it thrown onto her, oftentimes unfairly - she's the Slayer, after all; she's responsible for saving the world (a lot) - and that is something that increasingly marks her character through the entire series as the burdens pile on one another. With each burden, she takes up a larger amount of responsibility even when she shouldn't until it nearly breaks her in S6 and she makes the opposite, but equally unhealthy choice to behave in ways that can be considered irresponsible. It's part of her arc, and something that the series works towards in it's finale (Chosen).
In S5 this theme is emphasized not only by the need to care for Joyce in her illness and then care for Dawn, both in RL terms and from Glory, in the wake of their mother's death, but earlier on in Into The Woods (5.10), when she and Riley broke off their relationship after Riley blamed her for being "emotionally closed off" and therefore making it necessary for him to seek out vampire suck jobs (to cheat on her) in order to feel "needed"; a reading that Xander agrees with. Unfortunately, the show's writers seem to as well; in any case, Buffy's attitude goes very quickly from "who's fault is that [that you can't feel what I feel for you]?" to "I'm closed off so it's all my fault, I drove him away" (Intervention, 5.18).
I don't want to get into a detailed discussion of the events of ITW here because my point is only that the fallout from her breakup with Riley, her reaction to it, feeds into Buffy's sense of personal responsibility for everyone around her. (Besides which, discussing ITW any further would lead to a gushy, frothy rant and bring forth many "feminists words of rage", to borrow a phrase from gabrielleabelle.)
The important thing here is that Buffy's promise to Joyce tends to be overlooked in any conversations about The Gift, and I think that's a mistake. After Joyce's death, Buffy fulfills her promise to the best of her ability, and not just in saving her from Glory, but literally attempting to become "Mom":
"Who's going to take of us if I don't, Dawn? Have you thought of that?" (Forever, 5.17).
She comes to see Dawn not just as her sister but "part of me" in a way that would not have been necessary had Joyce lived. The show's writers then try to literalize this, unfortunately, with the nonsensical development that Buffy's blood will do just as well as Dawn's to close the portal. (Then again, BtVS has never been really big on the sense-making; and as we all know, Joss will throw logic out the window for the big emotional effect. But the entire premise of the show "one girl in all the world etc etc" is illogical to start with, so there we are.)
I won't argue that her promise to Joyce was Buffy's primary motivation; one of the delights of the characters on BtVS is that their motivations, as in real life, are deliciously and terribly complicated. Rather I would argue that it needs to be added into the pile, so to speak, whenever Buffy's sacrifice is considered by fans.