It was one thing for Grim to be on your shoulder when he was a cat. It was a completely different thing when a full sized human Grim was weighing you down. Leaning against your shoulder putting all his weight on you.

“Myahhhh! Being a human is so annoying. I can’t ride on my hench human’s shoulder anymore!” Grim complains leaning against you.

All because of one wrong mixture. One single mistake in the potion. He was now stuck as a human. Well at lest until Crewel could reverse it.