Depression

http://www.happinessinthisworld.com/2009/03/08/the-true-cause-of-depression/

Reminds me of an article I read elsewhere on the idea that despair is a natural response to discovering that a goal cannot be reached, developed to help a person stop trying, and move on to something else. Depression then, could be the result of a person being unable or unwilling to abandon the goal, thus causing the despair to stick.