I have had depression since I was pretty little, and now I'm at a point where I pretty much don't produce any serotonin. I know that serotonin replacements like Luvox (unsure if I'm spelling that one right) help because I've tried them, but I can't find a doc in my area who will prescribe them because it's pretty much the same as ecstasy. I probably would buy some just to feel normal for a bit, but it's always cut with bathsalts and things, so I don't think it's a good idea.
Anyhow, I'm pretty sure my depression was induced by pain. The reason for my thinking is that as a child I had crippling headaches and digestive problems that left me crying when I ate or used the bathroom that went totally untreated. I was also abused, so I'm sure that's some of it too. But I've never not been in pain that I can remember aside from when I smoke a lot of pot or huff paint, or get black-out drunk. It gets better, and it gets worse, but it never goes away. I don't do any of those things anymore because I can't afford the pot and I don't want the negative health effects of other things.
What I wish is that I had a mental health doctor who would address my actual issues, and could do both the mental health piece and the pain therapy piece so I can actually get better for once. I've got other mental health issues, but the depression just seems to be my monster.