#3 Bitterness because of the arrogance of the other
The reasons for which someone supports a football club can be really strong indicators about their own personality. More often than not, people support teams and adore players whose image they can identify themselves with, which makes them behave possessively with regard to their affiliations. Many of them choose to show arrogance, rather than pride in their choices. This leads them to come off as very disagreeable to fans of other clubs.
This arrogant behaviour makes them talk down to opposing players and team and results in animosity. Tensions are born, bred and accumulated with time, and the longer that fans of 2 clubs have been rivals, the greater is the degree of hate and ill-feelings that one faction holds for the other.