I know people who work in retail, and suck people exist. There are not a massive proportion of society buy even a bit of a percent is a lot of people overall. You would might that people out for a nice relaxing meal, or shopping in the middle of a holiday, would be pleasant to deal with: but you'd be wrong. I know a number of people in retail and I can tell you (through indirect experience) that the general public can be unrealistically demanding and completely unpleasant if their demands are not met, repeatedly, yesterday.

By way of an anecdotal example: I know someone whose father thinks there is nothing wrong with clicking his finders and yelling "BOY!" at a waiter to get their attention. Except in Italian places - there it would be "Luigi" rather "boy". I bet he consumes a lot of spit when he goes out for a meal...

For many reasons many people out there are dicks. I might well be one of them if I'm honest with myself, though not one of the worst by a long shot. And if you work in retail or catering you are likely to meet far more than your fair share of them. Remember: most workers in a shops or restaurants deal closely with far more members of the public in a day than most of us encounter closely enough to notice in a year.