1. In-group bias. This is the tendency to favor members of our own group and to view them more positively than members of other groups. When we identify with a particular group, we tend to see it as superior to other groups, and we may use negative stereotypes to differentiate our group from others. For example, fans of a particular sports team may denigrate other teams in order to boost their own team's status.
2. Social identity theory. This theory argues that individuals define their self-concept based on their membership in various groups. When our self-concept is threatened, we may engage in negative behavior toward members of other groups as a way to boost our own self-esteem. For example, fans of a particular musical genre may put down fans of other genres in order to feel better about their own taste in music.
3. Competition for resources. When groups are competing for scarce resources, such as attention, money, or status, they may engage in negative behavior toward each other in order to gain an advantage. For example, fans of different political parties may attack each other in order to gain control of the government.
4. Cognitive dissonance. This occurs when we hold two or more contradictory beliefs at the same time. For example, a person who believes that their favorite band is the best in the world may feel uncomfortable with the fact that other people don't agree with them. In order to reduce this discomfort, they may disparage other bands that people like in order to reinforce their own belief.
It's important to note that not all fans of certain groups engage in disparaging behavior. However, the factors outlined above can help to explain why some people feel the need to put down other groups.