Stupidity and/or moral outrage are the two things that bug the living shit out of me, online they seem to be the usual fuel that feeds the fires of idiocy. That and harbouring odd resentments over meaningless slights.
And almost every thread on every forum devolves into petty bickering sooner or later, which is I don't know what it is, but it's definitely something about the human psyche. Maybe dominance or suchlike. Back in caveman times, dominance basically meant more women to bang, that's been transferred into a weird abstraction online.