There are a lot of reasons people choose teams, I suppose. Pride in their hometown. Solidarity, to show they are part of the group and they fit in. A way to share in the accomplishment of an athletic endeavor.
When I was a kid growing up in the NY metro area, of course there were several NY teams to choose from. I decided I liked the Jets, Mets, and Nets because they rhymed. Then the Nets moved to Jersey and I didn't like them anymore because they were traitors. The hockey teams didn't rhyme, but I knew that the Rangers were my team. They just were.
I lived in Pittsburgh and enjoyed it, so I like their sports teams, too. I really don't care about baseball, but if the Pirates were in the World Serious, I would cheer for them. If the Pens ever play my Rangers, though, then it's too bad. Rangers all the way.
But really, the sport that gets most of my attention is college football. UFlorida is my alma mater, so of course I'm cheering for the Gators and I don't ever stop being a fan. The excitement and camaraderie of those games are associated with a very important time in my life. I loved college, and cheering for my school's team is a way of maintaining that connection. I don't feel the same emotional connection to any pro team the way I feel about the Gators. Hockey and the Rangers come close, but still not quite the same level.
Plus, I just love football

(31-10 Gators, 10 minutes in the 4th. Woot! Go Gators!!)