Basically you and Frisk done multiple Genocide run as Sans finally killed you and Frisk, after you and Frisk Reseted, you and Frisk done true pacifist route but Guilt... striked as you and Frisk ended yourself... with a knife right in Asriel fight... You have given up... forever, no longer can use Reset... as Asriel used your and Frisk's Soul fragments and other 6 human Soul, barrier broke... but... You and Frisk haven't come back from dead? monster Soul were back.. but.. your and Frisk's isn't... it's gone... like other 6 human souls... As Sans, toriel, papyrus, Alphys, Asgore, undyne woke up... They noticed... you and Frisk weren't in Flower bed... Especially Sans noticed it...

Sans thought: Wait... Where's the kid's? Uh... And there's no reset or load sound? Did they really gave up? Uh... I know that Asriel should have lose by now... Wait... Did Asriel win... Uh... That's strange timeline... Wait...

He then remembered a memory from Genocide timeline.

Sans's memory when talking to you: If we really friends, you won't be here.

As Sans finishes flashback...

Sans thought: Oh, gosh. I don't mean to them that they need to be gone forever... Oh, gosh... What if Humans start a new war... We don't have an Ambassador... I hope that Humans are ignorant enough to think we are someone else...

Time-skip after a year Sans woke up as usual... Everything repeats... except there's no you or Frisk... But... he noticed that Every Monster still looks for you and Frisk... It seems that Asriel thought about you and Frisk was permanent in all Monsters...