Papyrus's words fill him with an immediate revulsion - a feeling of dread and disgust that makes him physically recoil. It's a flood of emotional in direct conflict with his feelings of regret and longing towards her presence, a sense of nostalgia that fills him with desire and simultaneously memories of the terrible fear of being completely unable to stop someone from having their way with you bodily and spiritually.

That conflict makes it so hard for him to explain to Papyrus what's wrong. It makes it impossible to describe how badly that rationale hurts him, and how much it reinforces the bad thoughts he's been having already. Papyrus still doesn't understand. He was trying so hard to make the words come out and Papyrus still doesn't understand. He doesn't understand anything about him!

Along with a flash of irrational anger, he's left with a feeling of utter loneliness. No one understands what he tries to tell them and nothing he says makes a difference.

"No," Sans snaps - frustrated, desperate. He doesn't want her anywhere near him. He doesn't want anything from that place near him. New growls at Papyrus, sensing Sans distress and interpreting it as a threat that Papyrus is ostensibly causing. Times just sinks down against Sans's side hopelessly.