Waiting tables means you interact with all types of people all day long. Generally, you'll come across families, business people, and occasionally the rudest people in the world. You can tell a lot from how a person treats a service worker; the people who treat them poorly will have bad personalities 100% of the time.
I used to work at a Tex-Mex restaurant, which charges like $12 for a taco. I had a customer come in and ask for two gluten-free, cheese-free, and vegan quesadillas. I told her everything we usually put on it, and she said no. So, all she wanted was our gluten-free wrap with lettuce and tomato.
Okay, that is fine, but if I put this like that into the system, it will be $12 apiece. I can get you a lettuce wrap with tomato, which will cost $2.25. I was even going to heat it a little (the bread) for her. I tried to explain this to her twice, and she interrupted me and said, 'I know what I want. I want the quesadilla this way, and if you can't understand this, then go to the manager.'