Something about the change in Sans' voice finally clues Papyrus into the fact that this might not be hypothetical. That it might be...

"Is it because you're unhappy here?!? I can clean up more!" Papyrus pushes off the couch to sit closer to his brother again. He plucks the communicator off the floor, and tosses it back to where it slips between the cushions, its tinny sound muffled by the fabric. "See?"

But... no. That doesn't fit the way Sans is barely holding himself together, clutching at his shirt like that. "Or do you mean, you can't, can't. Like... you couldn't remember me."