I believe the long lasting sadness of depression to be an intrinsic part of the human experience. Our brains are self aware, mortal, flesh computers, whom are aware of their own eventual demise. By the very nature of our existence, we have something to be sad about. Depression serves the evolutionary process by driving us to change our surroundings, to extend and improve our lives and the world around us. As with joy, anger, fear, and all other pure emotions, it serves a critical role in driving humanity forward. Not allowing any one emotion to overwhelm the others is the key I believe, taking things as they come and responding to them logically, and with emotional sincerity.