I recommend the book Learned Optimism, written by Seligman, one of the guys who did the experiment that induced learned helplessness in dogs. He makes the case that depression ≈ pessimism = learned helplessness. He breaks down pessimism into three dimensions: permanence, pervasiveness, and personalization. If we interpret negative events/inputs as permanent, far-reaching in their consequences, and as totally due to us; and interpret positive events/inputs as temporary, limited in their effects, and not being due to us — then we are hurting ourselves. Within reason, the more optimistic we can be in each of those six ways the better off we are. Great book, not a difficult read but very solid stuff.