The monster's used to getting the side-eye, just on fact of being what he is... Let alone what skeletons represent to humans - and humanoid beings in general, he's discovered. The association with something that's dead and somehow lingering is, uh, not just gross, but he supposes it's why some humans have found his proclivity for humor and harmless pranks so off-putting.

Yeah, doesn't surprise him in the least that they have very different ideas of what's 'funny'. It doesn't seem to bother Sans, whose shoulders hitch in a quiet chortle.

"Lighten up, buddy, besides," he adds, "We're stuck here for who-knows how long. Might as well roll with it."