Liverpool-Utd rivalry is a modern thing and established by Fergie. Basically, if you look at it sensibly, Fergie's ambition in Scotland was to break up the Glasgow duopoly and make Aberdeen a credible force. When in England he saw LFC as the team with all the success. So it was inevitable that to be the most successful you have to challenge the best.
Maybe next year we'll stir things up with a mcfc-Chelsea rivalry debate. Perhaps we could say this goes back to the industrial revolution, and link it with "what Manchester does today....". Actually, we could easily claim a long rivalry with Chelsea, bringing up several grudge games over the decades, linking back to our Div 2 battles in 1983-84 and the fmc in 1986.