Cooking is good for bonding, and bonding actually has a purpose in this place. People aren't just another way of passing time in endlessly repeating days.

And it's fortunately impossible for Sans to damage himself - he knows, he's tried, though mostly just for scientific purposes. Maybe that's not the case for most monsters. It's never seemed polite to ask. But enough hostile intention to take away his single HP has to come from without, rather than within.

Sans would normally take a shortcut to the kitchens, but he's not quite sure he's up to them yet. Besides, it seems like an especially nice day out for someone who's just been dead. So he trudges along, a pace ahead of Frisk. It feels odd to lead, but then Sans considers he might actually know this place better than they do. It'd be fun to find out for sure sometime.

They walk in silence for a few minutes, before he can't hold himself back from asking the obvious question any longer:

"so, uh...gaster says they locked him up. the human who did all this."

He says it partly to reassure them, and partly to check their reactions.