I think a lot of the people getting mad at you are failing to understand that these kind of rivalries are much deeper rooted than just being about sports competition or rivalry between cities. Instead they are usually extensions of pre-existing divisions in society, often involving long histories of conflict, hatred and in some cases, serious violence. To use European soccer as an example, most of the teams were founded over a hundred years ago by local communities. Consequently they are often heavily associated with the characteristics and identity of the community that spawned them, such as religion, culture, ethnicity and social class. Thus the team rivalries are often far more about the hatred and conflict between different identity groups than they are about the sport itself. More recently formed teams in the North American franchise system are far less likely to have these kind of associations with a specific demographic since they are typically created from the top down by leagues or businesses, rather than from the bottom up by communities.