Part of it is a big difference in Italian fan culture and what they go to events for. The calcio (soccer) ultras, for example, are very different from English supporters, who are no less passionate. In England, they sing in response to the game, but it comes and goes and isn't nearly as coordinated and is more spontaneous as part of watching the game. In Italy, they sing non-stop from minute one to ninety one, have coordinated displays, tifos, plan banners for each game, let off flares, etc. The idea is more to put on a huge collective spectacle to theoretically inspire whoever they're cheering for, rather than actually sit down and watch the event.
It's not for everyone, and can be pretty intense, but having survived some crazy Serie A matches, it can also be a lot of fun if you go in knowing what to expect.
Where it goes too far is as
@Germax describes, where someone wearing the 'enemy' colours in the wrong place can sometimes turn to violence. Repsol being such an essentially Spanish squad (and Marquez being the guy who definitively stole Rossi's crown), would definitely mark you out, not dissimilar to wearing a Roma jersey in Naples, or an Inter jersey in Torino. Ultra culture is deeply embedded in Italian sport, even at the political level, and so change has been frustratingly slow. It is getting better, but there's still some hanging on to the old ways...