What Doesn't Kill You Makes You Stronger...
(WARNING: SPOILERS AHEAD. If you have not played the game or watched a play through, I highly suggest you do so before reading this.)
Sans knows what is coming in this reset. He knows that this is the timeline where Chara will finally succeed in possessing the cute, lovable human the Underground knows as Frisk. Papyrus hasn't been faced with Chara- and Sans is determined to keep it that way.
(Just saying, there is some light swearing in this.)