She's jumped from indifference to outright laughing at him, which is not appreciated. Not the sort of reaction you want when you've just revealed that you're a prince.

"Of course they can scratch," he answers, somewhat snappishly, "but I'm not going to catch lice, because I'll have a wash tonight. What sort of question is that? And they're frequently surrounded by people, but there are far too many people in this city for my liking anyway, so here I am."