On one level, sacrificing yourself for another is sort of the epitome of "doing the right thing." On another level, though, it is sort of needlessly wasteful and, perhaps, not always in the best interest of the world at large when the one who is making the sacrifice has more to offer than the one who is being saved.
It's like that famous question 'A building is burning down and everyone's trapped inside, who do you save? Five strangers or one sibling?'.
The right answer is 'five strangers'- to sacrifice the need of one (be it your sibling's needs or yours) for the good of the many. Five people's lives are worth more than one theoretically but when you actually think about that question, like really think and if you don't have siblings replace it with friend, parent, child, or any relative at all...and what's your answer? Deep down inside you know you could never let that person you love go just so you save a group people you've never met and in all honesty don't really care about.
Buffy was faced with that question in a very real way and her answer was, as you said Palabra, to hell with the world- I want my sister to live. I don't think she was taking her destiny away and I don't think Dawn felt she was either, because like her big sister Dawn didn't want a destiny or a higher calling, she just wanted to be a normal teenager with normal problems. She wanted to jump because she thought doing so would save the world, and therfore save her sister. The priority for both siblings was to save the other. And the fact that when this all went down they were up on the tower, alone together, isolated from the rest of the world, emphasized that the final decision was all about them. Not the world, not their friends, not the grand design but just them.
Buffy eventually being the one to jump- which Dawn doesn't stop happening, only mourns what she knows is coming- is because she's the older one. To paraphrase Dean on Supernatural She's the big sister meaning she's always right, always the one in charge and will never stop caring about her lil sis. Like any older sibling or parent (as theoretically that is what Buffy is to Dawn. There was a line cut from The Gift in which Buffy says 'She's more than my sister, more than my daughter'. This quote did confuse Xander in the original script....) she has to take care of her, and go to any lengths to make sure she survives, she's giving her the chance to live. And Dawn realized that. She didn't want Buffy to die, she didn't want her to jump to save her, but I think by the way she only stands there and cries and doesn't protest further shows she understood Buffy's actions and was deep down thankful for the gift that Buffy gave her. At least that's what I gather from Dawn's expression at the very end when she has come down from the tower and sees Buffy's body. She's all resolvey, serious and determined 'Live. For me' after all.
It's a paradox; by Buffy being selfish in not wanting her sister to die, she was selfless in the act of giving her sister the chance to live and the world a chance to survive.
And I think Death wasn't just a gift to Dawn, but the final gift to Buffy too as it finally gave her peace. So maybe it was
never Dawn's destiny to jump, maybe it was always Buffy's. Just a theory. Hehe
