What actually causes depression, and how is it treated?

Depression is the most common and in many ways the most debilitating psychiatric illness that there is. What do you know about what causes it, what the symptoms are, how it is treated and what the various treatments are? Have you ever had depression and if so were you able in one way or another to overcome it or are you still having some of it?
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Sorry that was a mess. What do you think causes what? In this scenario, depression is just as likely to cause the things that you mentioned as the things that you mentioned causing depression. At least I think so.
What actually causes depression, and how is it treated?
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