My theories on why people are assholes:

In general: People in childhood see assholic(i.e. bullying behavior) being rewarded by greater popularity, and there's no punishment for it, so they think it's a good way of getting through life. That many adults(while I am 19, I'm not a full adult) have just now figured out that bullies are the popular kids blows my mind. Like where were they during our entire childhoods?

In customer service: People hear the lie 'the customer is always right' and think that any behavior on their part is justified. I mean, when they throw a tanturm in the middle of the store, they get free stuff, etc, just to appease them. Also, stores are really crowded and people usually are in them after they have gotten out of traffic which makes them stressed and more likely to fly off the handle.