[It's not that Sans's isn't sympathetic - he had his moments of bitterness and doubt over the whole 'not being human' thing, but he's pretty sure he's come around on the concept since then. So when Rarity mentions she might be similarly inhuman, it actually comes as a sort of relief, which shows on his face just a little.

He hasn't spoken much with anyone else that has had such extreme bodily changes. After a few moments, his stare softens a little, and he makes a vague, accepting gesture.]

I think a lot of us are going to be in the same boat, that way. I know I am. It's kinda messed up at first, but it starts feeling right when you let it.