I'll answer, since I've actually gotten way under his skin on this before:
He's here for the attention, and he takes it rather personally. He's dramatically exited a couple times and re-entered because he flat out likes the attention. He stumbled upon a posting style that garners him a LOT of it, negative or not. people know who he is, there's a persona and a general knowledge of what "Matt" is, and he likes that. In order to keep that up, he has to maintain the posting style he fell into.
He likes the notoriety and he likes seeing his name on the board, seeing other people quote him. And then, reinvigorated, he goes back to the community board and puffs out his chest, proud of validating his existence on the internet. He gets a rush out of being the underdog, raging against the machine, setting up straw men to knock over to lessen the idea that he's wasting time here running in place. He likes the feeling of net-battle.