Sullenness of market cashiers

In my country, market cashiers are always sullen. It is normal for them to be like this because they deal with malicious people from morning to night. Sometimes I run into so many unstable people while I'm waiting in line. It's normal to have sullen faces. There are too many people yelling at the cashiers, getting angry and insulting them. Cashier is a very difficult job. It is very difficult both mentally and physically. They also get very little salary. Why would they keep laughing?