At least in part it's simple frustration and wanting to vent.
I know if I've been struggling and unhappy with a system for a long time, there's some part of me that wants to punch the system or the designers or the people who kept telling me 'you're approaching it wrong, the system is GREAT if you only do X...' or whatever.
It's childish, perhaps, but kvetching online about it is probably better than actually walking up to people and punching them for telling me how much I SHOULD be enjoying Frapple Ninth Edition, if only I wasn't such a carebear.
When you're frustrated, too, the small and unintended slights of the fans of whatever really drive you nuts.
Other frustrations:
'Why are you getting upset at me? I have no problem that you like the shambling byzantine mess of rules that appeal to munchkins and idiot children, that's totally fine! I'm just expressing my enjoyment of the clearly superior and wonderfullest game ever, and you're acting all wounded. Maybe it's the brain damage, that's a shame.'
So, yeah. I chalk it up to varying types of frustration and venting.