Rob steps back at the violation of his personal space. Even worse is the idea of having to make eye contact with someone else, which is almost forced by the situation. The source of his sudden apprehension will probably go misinterpreted though. He attempts to compensate by looking at Zuko's mouth instead of her eyes; normally he can do it so that people don't notice the difference, but if Zuko is paying extremely close attention to where his eyes are aimed she might notice what he's doing.

He frantically tries to remember what he was going to say before he finally noticed her scar.

"I-I don't just accept it. But I can't change it."