Buffy should have died her first year as a slayer, except that Xander brought her back, and the line of Slayers was diverted to Kendra and then to Faith.
What kept her still living for five year after that were the connections she forged - a Watcher more like a father, a mother who gave her unconditional love and support, but still acted like a mother, and friends who were willing to contribute their lives and their talents, plus Angel and his love and hers for him. Because of those connections, life for her wasn't just "the mission." It wasn't just slayage. It was nights at the Bronz and studying for exams and just being with these people. This is not news. This has been the running theme for those years - why she was different than Kendra and Faith. She had a *life*, or at least tried to have a life.
But, the theme of the last season on the WB, the season of Glory, was that these connections were starting to wear thin. Had worn thin. She couldn't bring herself to love Riley, and while she loved Dawn as a sister, that doesn't take the place of her mother - especially since she's aware of Dawn's origin.
She didn't die to save Dawn. She killed herself because she was ready to go. 20 is old for a Slayer, and five years is a long time to be steeped in death, especially when one of those deaths is your own mother, and when the only way to defeat the "Big Bad" is to kill a man who never harmed anyone.
Thank Joss for Giles and his lack of being a Hero. I love Giles.
And she was brought back and so on and so forth the last year and a half.
And now. Look at her. Look at our Slayer now.
She's deader than Spike. She has no affect, no sense she cares about anyone or anything, except possibly for Spike, and for fighting. All those connections that kept her alive are gone. Her mother died, her sister isn't real, her friends betrayed her in one way or another, if only by allowing her to be brought back. They've killed, they've left her to go someplace else, they've brought her back to death. She can't trust them any more. Except for Spike because Spike has never pretended to be anything but what he was.
She has nothing but the mission left, and she barely cares about that.