I don't think its that he's "happy", he just doesn't know how to live any other way. The world is entirely alien to him, people are alien to him, at least in this small little world he feels like he has a little control and can hold onto something no matter how small. He's not a guy aiming for happiness in the first place, he just wants his little corner to himself, that's all. He can't really handle the rest of the world. A girl offering him the world, cheap love, cheap thrills, the entire thing was alien to him. Even if you say he could have taken it, such a thing won't register to him to begin with. She didn't really understand that the money never meant anything to him in the first place, none of it did. The only thing he wants is this small hole to live and die in, never interacting with anyone, never stepping outside of his boundaries.