I was thinking the other day about how a lot of movies and tv shows have minor characters that exist basically to be curmudgeonly, and how often there'll be a very quick explanation of how or why said character got to be that way. Oh, don't mind Ed, he's been that way ever since his dog stole his wife, etc.
Anyway, I think a good backstory for a particularly sour character would be that he suffered a head injury in such a way that his olfactory system was thrown out of whack, and so he smells poop, constantly. Like that's his new baseline. It just always smells like poop. That'd put a pretty good-sized chip on your shoulder eventually, I'd bet.