Why are people cranky?

I know that I can be cranky, so I am not asking why is everyone but me cranky? I can be just as, if not more cranky than the next person. Why do we get that way? Why don't we just say what we think or feel and be done with it. It can be draining to be around people who are always cranky. I want to avoid them because I don't want to be bothered by it. Then sometimes I think it is funny because those folks seem to let the littlest thing make them upset or they become almost a cartoon character. I don't like being cranky. I know that I am not at my best or my nicest when I am cranky and I don't much care for that. I will say that it is difficult where there is no sunshine. However, that isn't the fault of anyone in particular. Here's to not being cranky.