I once read an article that said that depression evolved as a coping mechanism to isolate a member of the tribe from others in a time of great stress. With recent studies pointing to depression being hereditary, I think that goes a long way in saying why one would isolate themselves: in order to not pass on an undesirable trait of low coping mechanisms. However, today the population is so huge that depression is virtually everywhere. I definitely think it's biological and can be passed from generation to generation. I live in constant fear of getting in a relationship which might produce kids that also have the problems I and my family have, therefore I isolate. I don't really feel like it's spiritual or I'm "possessed," although depression has taken any spirituality I once had out of me completely.