Let’s say someone’s family was in the army, so they moved around a lot from birth, and they didn’t really have a ‘home,’ but they’re into sports—how do they choose their favorite team? Do they just decide to hold off until they move somewhere permanently, and then choose that city’s team as theirs?
It’s just one you vibe with. Maybe a friend or family member is a fan. Maybe it’s the rival of the local area. Maybe it’s a team from the first game you watched in person. Maybe you like the mascot (if they have one).
There doesn’t have to be a rhyme or reason, it’s just a game. Find one, follow them, you become invested in them, now you’re a fan. Some people take it more seriously, some people keep it light hearted, and some people don’t care at all.