If you're asking why people are so mean about the Twilight series, it's probably because A.) they don't understand it/like it and therefore call it stupid just like when a person loses a game against someone younger/less experienced and then say, "This is stupid!" or B.) they're the type of people to think that everything they like/hate is right and if anyone disagrees w/ them (even though it's a matter of OPINION) that someone is wrong.
If you're talking about meaness in general... see above. There's other reasons why a person is mean but I think those two are probably the most likely.
Honestly I don't see why people can't just mind there own business when not asked and at least try to get along.