In other words, Thomson and Andrews imagined depression as a way of forcing the mind to focus on its problems. Although rumination feels terrible, it might make it easier for us to pay continuous attention to our dilemmas. According to Andrews and Thomson, the mood disorder is part of a “coordinated system” that exists “for the specific purpose of effectively analyzing the complex life problem that triggered the depression.” If depression didn’t exist — if we didn’t react to stress and trauma with endless ruminations — then we would be less likely to solve our predicaments.

Wouldn't surprise me.