Sans... honestly, he's giving Kaworu's 'niceness' too much credit.

He might also be giving Kaworu's social skills too much credit, because Kaworu simply blinks, smiles, and says "I see," in a tone that's strangely knowing. A tone that suggests he sees something, all right. Something other than what Sans was intending.

What Kaworu sees is someone who is deliberately creating a constantly-shifting goalpost to prevent others from achieving a close understanding of him.