[ The arm waving irks him, but not more than the fact that she's obviously right. No one likes being emphatically told they're being stupid. "Just think about it" strikes a chord, too, thrusting him back in memory to his uncle's exasperated, similarly emphatic speech to him about acting smart, back in Ba Sing Se when Zuko had tried to steal the Avatar's sky bison, an idea that was equally blind as his escape plan now... Having thought he'd grown a lot more in the past few months since then, he resented being presented with evidence otherwise. Likewise, he's still embarrassed about having started an accidental fire because his temper lost him control of his firebending — and the refreshed shame in that doesn't help him keep his temper now, either. ]

Who asked you, anyway?! You asked to tag along with me — this is what I'm doing! Who's to say my plan to get back to my world would even work to get you back to yours? If you wanna waste your time doing homework instead of taking action, do that! There's a war going on where I'm from — my whole world's future is decided tomorrow! So I'm leaving today, whatever it takes! I don't need anyone else!

[ He doesn't mean to yell, Michiru. He's not especially aware of his own volume when he gets riled up like this. He's just scared, when it comes down to it. And doesn't know how to show any emotion openly except for anger. All his other painful emotions — and even some of his good ones — just get converted into anger by default. Desperation just amplifies it. ]