http://www.nytimes.com/2010/02/28/magazine/28depression-t.html?pagewanted=all

“Depressed affect made people think better.” The challenge, of course, is persuading people to accept their misery, to embrace the tonic of despair. To say that depression has a purpose or that sadness makes us smarter says nothing about its awfulness. A fever, after all, might have benefits, but we still take pills to make it go away. This is the paradox of evolution: even if our pain is useful, the urge to escape from the pain remains the most powerful instinct of all.