Good point, I guess where rivalries have their roots in old industrial disputes the teams need to be playing each other to carry it on.
Liverpool v Manchester started when they built the canal but obviously they've had plenty of other things to keep it going since.
My friend supports Spurs...
The West Ham v Millwall is to do with the dockers isn't it?
A quick google search throws up this - http://www.telegraph.co.uk/sport/football/teams/west-ham/6105500/West-Ham-v-Millwall-a-history-of-how-the-rivalry-started.html