I have no evidence but from being a sports fan for over 25 years I find people become fans of teams for two reasons. Either they are from that city|state|region or they like a particular player on that team at the time.

Being a fan of a team from your region gives you another connection to your community. It is something you all have in common. I would not call that silly. A little overly emotional maybe but not silly. It is just like the programming communities that we all fall into. I think we have all read stories on HN about someone leaving a or joining a language based on the community. Community can be important to some people.

And being a fan of a player, thus becoming a fan of the team, probably falls into your line of thinking as far as "picking a team for a mature reason". You like how the player plays the game so you want to cheer for him. If that's how you pick a team then that is fine as well. Some sports fans hate this way of picking a team but I think it is legitimate.