The rise of professional sports has cultivated countless sports fans, making sports viewing popular in people's daily lives. Deliberately cultivated by the league and its teams, the atmosphere of the battle between 2 parties on the field also extends to the mutual competition between different fan groups outside the field, creating a difference between you and us. In order to understand the influence of this distinction, this article aims to explore the formation of in-group favoritism and its related influencing factors through literature review focusing on the concept of in-group and out-group. Results indicate that in-group favoritism is mainly related to the maintenance of personal self-esteem. Fans' personal identification and the comparison results provided by the situation also play a role. Through the analysis of the formation of fan groups, the phenomenon of group favoritism, and the influence of individual and situational factors listed in this article, it is hoped that this article can provide valuable advice for practitioners and future research.