Lots of people complain that they get bad service at restaurants. The waiters who serve them get rebuked or glared at for messing up, when half the time it is because the customer made the mistake. Employees aren't perfect. They are human, just like the customers, and sometimes make mistakes. But they don't need their mistakes thrown in their face in a raised tone of voice in front of every other customer.
Customers feel like those serving them are beneath them because the waiters and cooks prepare and bring them food that the customer pays for. But without the employees, there would be no food to purchase.
Moreover, if a customer is mean it makes the workers angry and they often do not take as much care with that person's food. So the next time you are grumpy and are mean to a waiter or waitress, remember that they are in charge of your food until it reaches you.