Because if something happens which a particular person doesn't like, aren't they allowed to voice their opinion on why they don't like it. Eg. If someone doesn't like the fact that sasuke wants to kill the kage amd start a revolution, aren't they allowed to say negative things about that character?
From this there will be someone trying to defend what sasuke did and will argue against the first guy. Arguments will get heated to a point where the two parties come to dislike each other and therefore hatred is born.
Obviously to make up with your enemies you have to find other things that you have in common bit in a maruto forum,the most talked about topic is naruto and sasuke rivalry most likely if anyone is part of one fanbase, they would dislike the other.