Dave Barry said it first, but I read it and thought, "I knew that!"
In his book, 'Dave Barry Turns 50', he talks about fifty things he's learned over the course of his life. One of them is, "If the person you're having dinner with is nice to you, but rude to the waiter, they are not a nice person."
And it's absolutely true. There's a certain class of person (and you know who you are) who thinks that being in a restaurant gives them a license to be rude, demanding, obnoxious, and cruel...and all to someone who has to respond to that with nothing but politeness or risk losing their job. Sure, we've all had situations where the food is bad or the service really is terrible, but you can find ways to let people know that without taking out your frustrations on them. (Especially the food. They didn't cook it, they just brought it out to your table. When you shout about overdone steak, you're not even shouting at the right person.) Really, you can tell more about a person by how they treat the waitstaff than anything else about them.
I'm gonna let you in on a little secret. When you die, and you march up to the Pearly Gates, God's going to have a big book with your life in it. And it's not going to contain your donations to charity, or the number of times you swore or had sex or got drunk. No, that book's going to contain a list of every time you went out to eat, and exactly how you treated your waiter or waitress.
God knows how much you tipped. Literally.