When I was younger I think I was depressed. It got to the stage where people would talk to me and I wouldn't talk back, I'd just stare; I'd lie in bed doing nothing; I let my hair grow long because I didn't want to go outside or talk to a barber.

I'm much better now, and I think exercise was one of the key things that improved my general mood. It was a constant sturggle though for a while.

YMMV, of course, and I was never diagnosed or anything.