Rinoa is both horribly amused by Zuko and in the middle of feeling really sorry for him. It's obvious he's arrogant, but it's also obvious he's earned at least some of that arrogance, and even though he's kind of a brat he's definitely not a bad guy. He's just got a short fuse, and she can't help poking that sort of thing. But with everything that's been happening to him lately, she kind of just wants to give him a hug, and she very well might if he starts looking too sad for her tastes. Which he will of course hate.