Hahahaha at ALL of you who think it has ANYTHING to do with football.
Trust me, I've been on that side of the fence and I know that the football has little to no bearing on it. You get two gangs of angry young men, who have no positivity to look forward to in their lives and who have become disaffected with society/government and they've got a reason to fight, by jove they'll scrap!
If football didn't exist, I'm sure these groups will find other reasons, such as fashion sense (see Mods vs Rockers) or "Territory" (see US Gangs in 80's)