I work with a guy who is the most negative person I've ever met. I know him quite well now and he has no significant family/childhood/health problems, but will merrily admit he enjoys complaining. It's relentless and he gets himself really wound up about nothing, and can rant about the weather or customers for hours. It's like it's a deliberate personality trait and I can tell he's hurt if I am short with him or leave the room but I CANNOT TAKE IT when he does it and I am unwell or in a bad mood myself. Isn't it polite to make small talk that isn't angrily raising your voice? It's got to the point where all our colleagues avoid him and I give one-word answers as much as possible.
How should I deal with him?