Why do MMORPG communities always suck?

I have played EVERY single popular MMO known to man (both free and pay-to-play) and not one of them had a decent community. I remember playing this one small MMO which had only had a few hundred players and it was alright for a while, but once the game was advertised more, then a lot of shitty people came and ruined it. Why?

The community sucks because of one thing:kids.

Immature kids ruin games because they act like they do in halo games.

There is nothing wrong with saying:pwn u nub or something like that,but have the decency to not trash talk people for wanting to enjoy the game and disrupting their otherwise fun.