"you're useless" "lazy" "you're incompetent and... ineffective" These insults wouldn't normally affect Sans in any way.. but they do Papyrus has turned his back on his and.. Sans doesn't know what to do about it. He has tried to impress him but he can't. He's even given up saying puns. When he builds up to it Papyrus glares at him angrily, which causes him to get quiet quickly. By this point he's given up trying.. so he's going to take a different approach.