Being a football fan is akin to belonging to a religious sect. As a Leeds United football fan, it is your duty to hate the team from the other side of the Pennines - Man U!To Leeds fans they are also know as "Scum". This (I presume) links back to the war of the roses, a conflict between the Houses of York and Lancaster.The rivalry in football terms stems back to the 60s when both teams began to be successful. The fans have always hated each other, regularly chanting offensive songs. Leeds' fans have a particularly distasteful song relating to the death of Manchester United players in a plane crash in the 1950s.
Now, personally, I don't like all this tribalism. It's good to have rivalries to compete with but the difference in the clubs is marked, Manchester United being one of the biggest teams in the world. They have many big rivals such as Liverpool, Manchester City and Chelsea as well as rivalries with big European teams. Leeds and Man U. have a long history of players who have been transferred and played for both teams:
Johnny Giles
Joe Jordan
Gordon McQueen
Arthur Graham
Denis Irwin
Lee Sharpe
Gordon Strachan
Eric Cantona
All these players have been great for both teams over the years.
Also, personally I've lived in Manchester and met several City and United fans. They've all been great with me and Manchester as a city is a great place. The rivalry also springs from jealously of Man United's success. They've always been the most popular team based on attendance and they are now the most successful club in England. So for me this rivalry doesn't concern me.