Just to clarify a bit, I wasn't having a bad day when I put up that last, just being struck by just how many of my friends have struggled with depression at one time or another in their (relatively) short lives. Of course, you could also say that since I suffer from bouts of anxiety disorder, I am more likely to feel some friendly bond with people who are prone to it too. Good point, but it's not just my friends I'm talking about, it's a documented trend.
K'wyn put it rather more sensibly than I tonight: for most of us the basic needs of food and shelter are pretty much taken care of, we don't have to hunt/gather food and keep off the sabre-toothed tigers, nor do we have to slave all day in the mines (at the risk of our own lives) to put food on the table. So we are caught up by more ephemeral essentials like "making something of yourself" and such like. I've read a similar analysis of why Freud came to belive that the whole world was sex-crazed. His patients were well-off Victorians, the only thing they were really deprived of was sexual freedom, so more often than not it became the seat of their neuroses.
OK, rant over. For now.
muah hah hah