I think there's a whole thing about happening to be born near a good football club, staying there, growing up a season ticket holder, watching quality football, then judging other people for supporting your club.
To give my United story: I grew up seeing my dad, who was born in Salford, every other week. He'd moved away from Manchester as a teenager but was obviously a United fan by then. Early in his career he moved further away still, met my mum, had children, got divorced and moved further away again. Obviously he didn't change his club allegiance upon any of these geographical moves. My mum wasn't going to take me to local football so sometimes, when something could be worked out that involved him still getting to see my sister and he could get tickets, he'd take me to United. Despite living hundreds of miles away, my first football match was United vs Wednesday in 1993. How could I ever turn back from that, even if I wanted to?