It's not only the question that bothers him, but the way Sans phrases it. The way the skeleton doesn't react, the way he's always smiling, all of it plays a part in pushing Psi forward. He steps forward, stomping his foot on the ground as he does so.

"Yes!" What kind of question was that to even ask him? Wasn't it obvious. "What do you even know about it! You're not a troll."