Who says that supporters have to support their local team?

The argument rages that the likes of Man Utd, Chelsea, Arsenal, Liverpool and some European teams like Barcelona, AC Milan or Real Madrid have a significant contingency of supporters who don�t originate from said cities. Who says though, that we have to support our local team? What if somebody comes from Loughborough (right in the middle of the triangle consisting of Leicester, Nottingham and Derby) for example. Which team should they then support? Or cities that have two teams like Manchester, Sheffield, Liverpool or London. Glory hunters aside, there seems to be no reason from my perspective why one should support a team just because you were born or grew up in the city of the same name as the team.