Why do people have to be so mean about other people's opinions?

Usually because:
A: They want to get a reaction.
B: They think they're cooler or better
C: Some of the nicest people can get carrieed away.

Because they are convinced their own opinions are correct and they want to prove that by saying things against someone else's opinions.

People have massive egos and if you tell them they're wrong well they're aren't gonna like that very much. Opinions are very personal and almost sacred to some people so disagreement isn't welcome.

Because they fell better about themselves by making other people fell horrible :c

Haters gonna be haters but I don't care, people have their opinions and though I may not always agree with them I won't judge them by that. if someone liked something that I don't then Im not gonna be like 'oh that's for losers' or anything like that.I respect their opinions but some people don't..