First step is to forgo a distinct personality.
A fascinating discussion when you live in the arse end of Ireland and discover that most people are fans of this team or that team… all of which are English.
Like a religion, or culture. As other have said, location, family, etc. Also, advertising plays a role, and then there are people who get really into it and spend a lot of time focusing on different teams, and sports, until they just become team/sport agnostic or focus on the one that meets their statistical, and business practices, requirements.
Geolocation or affinity to location, significant player from region playing at team, family inherited “not ok to not like team xyz”, stats, position (underdog, champion) etc
If it’s your city’s/country’s team, has historically been your family’s team, there’s a player you particularly like or there’s something special that makes the team stand out (eg I support Chivas/Guadalajara becuase they only hire Mexicans). It has little to do with sports, tbh. I know very few people who support a sports team on its sporting merits alone and they’re all insufferable.
