Sans is often, and often intentionally, frustrating. No wonder it's hard to think of particulars. But there's no sense asking Papyrus what the cause might be. Right this moment, frustration with his brother is less ohis mind than a mix of frustration and confusion with Lambert.

"What? Sans, minding me? He's the one who keeps secrets, and breaks laws." Papyrus steps out of the way of a pair of unfamiliar dancers, and scowls as they pass. "We're both adults. I can't clean up all his messes... but I can try to push him to do it himself."