I'm pretty sure my depression is basically due to brain chemistry issues. It seems to come on for no particular reason. Medication isn't an option at this point, for various reasons.
But I have noticed that if I take a vitamin B-complex supplement, it helps a bit. I'm not 100% sure of that correlation, but so far so good.
When you're really depressed, though, it's hard to do much of anything. I have a lot things that I enjoy, which can help me feel good. If I'm only a little depressed, for instance, going for a walk can perk me right up. If I'm very depressed, though, I get to the end of the block and think, "Oh, god, I hate this, I just want to go cover myself with a blanket and go to sleep." And in that state, you don't want to be near anyone, in case they say something really awful, like, "Life's what you make it!" or "turn that frown upside down." If those things cheer a person up, then they weren't really all that depressed to begin with.