Bedrockgames
I post in the voice of Christopher Walken
Think it is like a sports team, people just get behind it and want to see it do well, that or mob mentality. Just wonder if humans release a biological agent that effects a group to the same experience making a social bond.
I think it is pretty ingrained in human nature and it isn't just sports fans and movie enthusiasts. People do it for politics, for RPGs, for their favorite technology, philosophies, etc. It isn't all bad. It isn't all good either. Somewhere between online fan drooling and the Nika Riots, people can enjoy a shared experience of awe and wonder together in a movie theater or a celebration of victory after an exciting game.
Sometimes I think it actually has to do with morality. It seems like we imbue the things we like with moral value (i.e. it isn't just star trek but the values star trek represents, it isn't just Manny Pacquiao but the values and qualities he represents to people). That can get dark (like it did here in Boston in 2004 when people tore up the town after the game and a women ended up dying). But it can also bring people together in positive ways and even give them a channel for some hope (like Ali did for a lot of people when he was fighting).