I've never seen a deeply plausible argument for internalism

If we all agree that some different moral statements are motivating in different amounts, the burden of proof is on the one who says that a certain amount of motivation is impossible.

E.g. The belief "It would be nice to help a friend by helping carry their couch up the stairs to their apartment" makes me feel mildly inclined to help. The belief "It would be really nice to give the homeless guy who asked for food a sandwich" makes me significantly inclined to help. Why would it be impossible for me to believe "It would be nice to help my friend with his diet when he visits me" and feel nothing at all?