Because you depend on tips

You get paid minimum wage, plus tips.

The minimum wage barely pays the rent, so you’re dependent on the tips.

You resent having to be nice to your customers.

You’d be nice to your customers anyway, but because you have to be nice to your customers, you resent it.

You have to be nice to your customers no matter how nasty they are to you.

You can’t help but feel a little humiliated to be dependent on other people’s generosity.