I remember when I worked as a waitress it seemed some people came in only to have someone to take out their frustrations on.
It had nothing to do with the food or the service, they were bound and determined to give someone a hard time and we happened to be available… and powerless to do anything about it.
I had a guy complain to my manager that I didn't greet him cheerfully enough when he came into the store. Then he proceeds to vent about government and taxes for twenty minutes to that same manager.
He didn't even buy anything.