Because it's just not enough that J is responding to Sans's accusation of dimensional bias by mimicking the same thing back at him - no, this goes deeper than that. The words that immediately come to mind upon hearing that part never get the chance to go to his mouth based on what comes next. Hoo boy.

Deja vu of deja vu. Half remembering things from what feels like another time. So many things have been done to him and he's never been able to stop any of them.

"I thought I remembered you," he says, and though the some of the volume and anger has left his voice, there is something darker and sharper rising from beneath the surface. All the light has left his eyes, besides the burning light of magical energy in his left socket.

He steps forward, and though his mind is elsewhere, the magic contained within his small body is starting to leak out into the environment. Something is broken, there, and Sans doesn't know how to fix it - and it's probably a safe guess that despite declaring himself the authority on the matter, J doesn't know either.

"So, touchin' a guys soul is no big deal if he doesn't remember it, right? And hell, why bother asking for permission when he's physically present, either? Pretty sure that guy's head isn't all together, anyway! The fuck do I know about my own soul?"

All at once, the gravity in the room slowly starts to shift. Object start to become light, enough that they can float. Small items will begin to head towards the ceiling while wrapped in a bluish glow.

Whether or not Sans notices is unclear, but the derisive laugh he lets out indicates that he probably doesn't care right now.

"Did'ja ever stop to think that maybe some things are better left forgotten?"