Depending on the one purpose thing...sometimes you feel all the responsibility.
[Buffy used to live a guilt free, responsibility free life and then she became one thing, the only thing that would matter ever again: The Slayer and because of that, she had all the guilt and all the responsibility. If the world wasn't right, she needed to do something to change it, make it better, make it safe. If someone got hurt, she needed to be faster, stronger, better because if she had been any of those things, no one would have been hurt.]