Board Thread:Questions and Answers/@comment-27491885-20151228162741/@comment-32984195-20170829190544

When you meet Sans for the first time, he recognizes you as a human. Anyone would say you're not the first human he sees or, at least, has more knowdlege about them. He tells you that his brother has never seen a human before, and that it'll make him happier to see one. I guess Papyrus is the younger one, not aware of what does King Asgore do with the humans and their souls, still thinking he will kindly invite you to have a cup of tea before letting you leave the Underground save...

Of course, it could be just that he's extremely innocent and no one has ever told him the truth. But I don't think so... I think he's still just too young. And there's no use in telling him, it would just make him unhappy.