I don't think the decision has anything to do with Buffy being selfish. I think it has more to do with what she believes to be right or wrong. In season 7 when Giles presents the slayer with the question again as to whether or not she would sacrifice Dawn to save the world she responds with "if I had to". She at this point understands that the "mission" is what matters and as much as she loves Dawn, it would have been the right thing to do to save the world by ending one life..... or something like that, wow that felt like rambling
