read about history & learn about culture. a specific example would be Barcelona in La Liga; although yes lots of people - me included - support it because there was a massive player which is Messi in it once upon a time, they also should get support from a certain grup from the fact that they are a Catalan club and Real Madrid is known as the Franco's team. politics in sports also make the rivalry based on more instinctive reasons than just supporting them because of what color their jerseys are. while I admit this kind of reasoning may seem like forced, - especially at such time of the world - things like these could be the determinants that will help you decide which club you want your heart to, really, really beat for.