When I was growing up in Ireland in the 70s and 80s, you supported 1 of 2 teams; United or Liverpool. Somehow, you just became bitter about the opposite side. No one really understood the depth of the rivalry or where it came from?
Even today with younger fans I feel that many don’t get it when they list Chelsea, Arsenal etc as our biggest rivals. City? Local neighbours and local rivals but not the same bitterness. Leeds come close but not the same.
Liverpool fans were a special brand of obnoxiously arrogant braggers when I was growing up, even adults, looking down their noses at anything United. I still didn’t understand it but bitterness grew.
This article does a decent job of explaining where the rivalry first grew from