Matty's not co-dependent, exactly, just very attached, and he's coping, and it's not like anything's happened, except so much happened all at once, because George is allowed to have girlfriends, because Matty and George were never exclusive, they were never anything really, it's just that Matty would like to think that there was something, and that that something was more than wishful thinking, but he reckons it's too late for that now.
And it was bad enough seeing George kiss her. Maybe it would have been fine if he'd just reacted like a normal person about it, but no, Matty didn't speak to anyone for days, and George moved out to live with her, like Matty can blame him, and Matty's had what is easily the most depressing four months of his life, because it's come to the point where he's pretty sure he hates George, except, he doesn't, not really, because it's Matty's fault, if anyone's, even though he's yet to get his head around it.
But then before Matty really knows what's happening George has broken up with his girlfriend, and needs somewhere to stay for a while before he can sort things out, and Ross is just kind enough to volunteer Matty's house - he has a spare room, after all. It used to be George's room, after all.