Sans is somewhat aware of the whole save and reset feature -- billed in-setting as timeline shenanigans.

people close to him that he's lost due to the resets, be it in the physical sense or the they-no-longer-remember or aren't-the-same-people sense. The poor fella's at the point that -- as he mentions in his fight's dialogue -- he's given up all hope of getting back to them. The room you find the pictures in has the evidence of his attempts to, from what I recall.