For starters, the very job title describes how tough a waiter's job is. Someone who 'waits on people'. It can be tiring and annoying and frustrating and, for the peanuts that they get paid, they show a great deal of patience while putting up with all the shit customers give them.
We might know many people in our lives but respect is something we save only for a few. Waiters, however, have to show respect to every customer that comes to their service.
Whether it is working morning shifts, night shifts or holidays, they learn to work with the same people day-in and day-out without complaining. Monotony does not hamper their level of commitment.