"It was the right thing to do." Simple as that. Besides, Raylan was the one who had invited Doc back with him in the first place. Even if Doc didn't care enough to want to share a space with Malcolm, which was certainly not the case, it was Raylan's stray-collecting tendencies that brought them together in the end.

"You have already helped me plenty." Doc will be feeling a lot better about settling down in the schoolhouse when they're all supplied up and ready to bunker down for at least a few days without having to leave the premises.

"Don't suppose you'll be needing any more willow bark, with your pills on hand." Not that Doc is in any condition to go chipping away at anything, although Raylan can probably help with that particular fetch quest, if needed.