In John Updike's short story "A&P," Sammy calls the people in the store sheep. Sammy refers to the people as sheep because he fails to see any differences between them. Essentially, he does not look at them as individuals. Instead, Sammy thinks that they way they walk aimlessly up and down the aisles and through the checkout, without any true thought process, show them to be like the herded animal which simply follows the crowd.
This idea is supported when three girls walk into the store wearing bathing suits. Given the girls are not dressed according to the norm (for the store), they are thoroughly embarrassed and leave quickly. The girls do not follow the unstated rules and are not considered sheep by Sammy.
Anyone who shops in supermarkets (which includes practically everyone) must often get the impression that the cashiers have very little regard for them and consider them just faceless figures to be processed and gotten rid of. The shoppers have to funnel through the checkout aisle like sheep being processed for one purpose or another, either to be sheared or disinfected or herded onto railway cars. Sammy obviously is too intelligent for the kind of work he is doing. He is college material and only working to make a little money for school. Naturally he would come to hate his boring job and--especially after having difficulties with the many difficult customers to be found in any supermarket--would come to hate customers in general. The pretty girls in bathing suits are, of course, an exception. They not only appeal to him on a physical level, but they remind him of the greater world outside the supermarket where people are enjoying themselves in many different ways, including swimming and lying in the sun. Young men like Sammy are often unpleasant to have to deal with at checkout counters. They often seem to have a characteristic "attitude" of boredom, resentment, and superiority. They never intended to be cashiers wearing aprons and doing the same things over and over again. Sammy can't see himself, but he may be like that himself.