No one is born strong, not even monsters. The pain may not only come from others. Sans wanted to give up more than once. Whether it was Gaster or Frisk, their appearance and disappearance only told Sans that maybe he really couldn't protect anything. Ppayrus is everything to him, the only relative he cares about, but it is also the obsession in his heart that he is destined to pursue and keep.
Concept: Undertale AU, Underfell fanfic