You had fought sans many times before in genocide. But he made you and Chara realise what you did was wrong and you reset back to the beginning ... going out to snowdon meeting sans again but hes forgotten the genocide runs

you turn around before he said to and shook his hand

Sans: haha! Old woopy cushion on the hand trick... hey... your not laughing... have you heard that joke before... your... human... right kid? You look sad ..