As described by a recent article on scientific american... depression may be the evolutionary approach to serious analytical problem solving?
Theory... we get depressed because we have a problem we need to sit down and solve. Ruminating over it directs our energy/resources towards this problem. Then once the solution is found, the depression goes away. If this is true... does it point to cognitive therapy being a "better" treatment than antidepressants? That is, resolve the problem rather than covering up the symptoms?
Depression's Evolutionary Roots: Scientific American