It's like you said, geographical, but also it's whatever players you see in action and like for whatever reason. For example I'm a Raiders fan just because of Bo Jackson. Over the years they have sucked, but I still find myself cheering for them.

I like Notre Dame for college football just because my older brother and our friends liked them. Part of it is also parental/family/peer influence.

Baseball I like the Blue Jays because they're the only Canadian team.

Basketball I go for whoever is playing against the Heat. lol
Actually I love the Bulls because of the Jordan era, and now I like Derrick Rose.

Hockey I go for the Canucks because I live in Vancouver.

So yea, I guess its a big combination of geography, preferred players and family and friends influence, at least in my case.