It's probably the fact that Finn has never actually tried a guilt trip on him that makes it so effective - he doesn't expect it, so he doesn't suspect it for the ploy it is, and instead visibly wilts a little. It had been all fun and games until he actually felt bad.

"You know that's not the problem, Finn," He said, no longer squirming. "It would be a lot easier to spend all day in bed with you if I could actually move around while doing it. Trust me."