As all the major pro-sports leagues have been plagued by the constant trading of players like so much cheap merchandise, the one bit of constancy we can see year in and year out is each city's fans. Fans generally don't switch their allegiance from one team to another, unless something egregious happens, such as a worthless idiot like Dan Snyder buying the team and causing a long period of deep sucking.
However, before you decide to adopt a hockey, baseball, football or basketball team as your new favorite, keep in mind the unsavory company you may be joining in terms of your fellow fans:
1) Boston area fans. These townies tend to be unduly aggressive, building up the sort of frustration that comes from a 90 year lack of World Series victories. They may lash out unexpectedly at you, so watch out!
2) Cleveland area fans. Be extra cautious around these folks, because they are sort of divorced from reality. Their long beloved Cleveland Browns went and moved to Baltimore, which says a lot when Baltimore is a step up from your city. Then, rather than going with a fresh start and a new team name when Cleveland got a new franchise, the city opted to stick with the name "the Browns." This is like your girlfriend leaving you, and you decide to call your new girlfriend by the old girlfriend's name so you don't have to change your tattoo.
3) Detroit area fans. These people actually don't exist. The people you see in the stands at games are all there just to ask for directions for how to get out of Detroit.
4) Pittsburgh area fans. Be careful around them so you don't get run over by their overwhelming sense of smug. It is impossible for a Steelers fan to have a conversation with you without slipping in the number of Super Bowl victories their team has had. If you ask them how baseball's Pirates are doing this season they will change the subject back to the Steelers.
5) New York area fans. Don't point out that their two football teams share a crappy stadium out in Jersey. They've heard it all before.
6) Los Angeles area fans. These people also do not exist. Everyone in Los Angeles is from a small farm in the midwest, trying to make it in the "business", they just have to get a producer to read their screenplay. Any team loyalties they may have would be towards the Denver or Kansas City or Minneapolis teams, but they don't really follow sports anyway.
7) Washington area fans. These bloodsuckers will root for whatever team they got free tickets to go see.
8) Philadelphia area fans. This is the lowest form of fan. They are violent, criminal, and nasty. Remember that scene in Rocky II when Rocky was running down the street with all the kids following him? What they left out was where the kids caught up to Rocky and beat him to a pulp and stole his wallet.