For quite a period of time I was really, really happy... with everything in my life. That's when I moved to the city. Since then, any number of things have contributed to me being depressed. My return to drinking, smoking, parties, financial woes, etc., have all contributed to me not being happy with myself... so I do them more because I'm "depressed" or just simply not happy with myself, which only makes them feel worse. Ironically, I know these things, but have no initiative or drive in me to change them (or anything else).
Depression is an end unto itself. I know I can fix them. I know I have the power. I just don't have the energy.