It totally catches Sans off guard, how that's the facet of the situation that Papyrus zeroes in on. It's obviously painful for him, yeah - even with only half formed memories, it's almost been enough to plunge him into misery. But... who cares about that? He's not the one who disappeared, is he? He's the one that never used the knowledge he had to fix anything.

He was the only one who could do anything to save him and he didn't.

He shouldn't be surprised by Papyrus's compassion, but he is. Mostly just because he keeps feeling like the focus shouldn't be on him. Like it shouldn't ever be, and how just staying in the background and doing his thing had been so much easier.

"It... happened years ago," he answers, uneasily. "Things only got more complicated, I just... things are more messed up than they seemed like, and nobody else needed to know about that. You didn't need to. And even when I told you about it... I don't think it ever stuck."