I've never quite understood it myself. I really play games for challenge. Getting cut down in 1 hit and cutting down in 1 hit is boring, uninteractive, and usually a waste of time. I mean killing gray kobolts and looting their copper surely should be the same thing right? Not only do you get to feel superior to something gray but you get copper too.
At the risk of sounding as edgy as the other guy I've only ever seen catharsis(I got killed leveling so should they), thoughtlessness(cause can), role-playing(red = dead), and sadism(something about masturbating to tears) offered as answers. Which pretty much sums up almost all of the responses in this thread. Of course the role-playing one is the only one that I feel serves a legitimate purpose. And of course those that do it out of sadism (few and far between imo) are usually pathetic but they will stand out in your mind as they camp you for 20 minutes.