I have a rule in life; treat me nice. It's not uncommon to stumble across another person who is not having a good day, in a bad mood, and is not being nice. I don't think we should let that stand. My solution is to I say, "why are you mean to me when I'm so nice to you?" Their response? Usually, a silence that is quickly followed by confusion. They realize what they are doing, that I am correct and apologize. Life is good again. When they keep up their bad ways, I say something else. "I don't deal with folks who aren't nice to me. Let me know when that happens." Then I leave. It works like a charm.
