Plenty of reasons.
To begin with its a lifestyle choice, the casual type lifestyle. That appeals to many people, especially if they grew up around it and gives them a sense of identity and essentially gets them into a tight knit group which makes them feel important. Also when fighting you are essentially putting your life in your friends there with you in that they dont run off and leave you, i guess when you are fighting with your friends you would create extremely close bonds, which is probably attractive to those with little family or just those who want that.
Then there are some guys who do just genuinely like a good scrap, and football hooliganism is a easy way to do this whilst only really affecting those who feel the same.
But i would guess the biggest reason is the 'buzz' you get, every interview i've seen they have mentioned the adrenaline rush you get and how it is very addicting. Makes sense i guess considering many soldiers back this up saying the adrenaline when fighting on the front-line can be addicting, and some soldiers do end up becoming hooligans to try and re-live this buzz. Saw one interview where one bloke was able to give up cocaine but was not able to give up hooliganism, and another where a guy said if he was offered a gorgeous girl or a good football fight he would go with the football fight. I suppose its in a way just a extreme sport, people can get addicted the the adrenaline rush it gives.