The people, the challange, reaching 60, aquiring gear, helping others, taking out the horde that tried to gank me and to escape some of the hum drums of life.
Because it gives me something to do with my excess time. I don't have any of that anymore since college started pwning me, but by now I've got so much time invested in it that I should probably see it through until I hit 60.
cmon ppl you all know your lives are boring or at least more boring than the game or you wouldnt play. lets get those honest votes going and have "Escape my hum drum boring life" take the win ;]