Despite Papyrus's attempted leniency in his suggestion, Sans immediately goes tense, his body curled and defensive.

"I didn't dream it," he snaps, like it's an argument he's had a million times and he just refuses to deal with it anymore. He looks away from Papyrus then, real frustration showing in his eyes with a harsh brightness. The flash of anger surprises even him - especially when he can't really remember being angry at Papyrus for at least a solid decade.

He knows it isn't fair, that it's out of place, and that reality immediately just makes him feel ashamed of himself. This isn't Papyrus's fault. It's never been his fault. He tries to force the hurt down, staring at the ground, his expression softening as his immediate remorse settles in.

"...I used to wonder," he adds, hoping to soften the response a little. "But... it wasn't a dream. I have proof of that now."

That much is invaluable to him, for all the pain it's brought him.