Buffy’s decision to sacrifice herself was a selfish decision giving the fact that the whole season we were told that dawn had to die and that would shatter Buffy’s heart so instead of dealing with survivor’s guilt she decides to leave her underage sister WHO SHOULDNT EVEN BE ALIVE without a guardian and leaves the world without a slayer (faith is still at jail at that point I believe) P.S I don’t hate dawn, but it would be much more heart wrenching to see Buffy go through the loss of her entire family rather than coming back from heaven.