As one who falls into lengthy periods of negativity myself...I would strongly advise you to stay away from your negative friend. You won't be able to cheer him up no matter what you say, and he will probably drag you down into the hole he's in while you're trying to. That kind of misery feeds on itself.

When I'm in one of those negative times, I mainly just avoid hanging out with other people until it passes. I become a hermit. I am not that eager to share my misery with my friends. I've known a few people who were just about always negative...or very frequently so...and I avoid them if I can. They are far more than I am able to deal with.

Professionals get paid to deal with that stuff, as do some other people such as priests. If you get paid to listen to it, fine...if you don't, you're probably better off without it.

As Abe Lincold said, "Most people are about as happy as they make up their minds to be." And that's true. Funny, though, it doesn't seem true when I'm in the grip of depression! Depression is a self-justifying and self-promoting state of moderate insanity.