"It's not for me," he said. "I might rule a country one day."

Big emphasis on might. Even if Aang's arrival has prompted Zuko to change his mind about being so certain of his impending death, he wasn't quite certain he'd ever wind up on the throne. And, more importantly, he didn't want to. He supposed he would, if it came down to it and there weren't any other options, but in his mind, his uncle was the rightful heir to the throne. Not him. Who better to be Fire Lord in the wake of Ozai's madness than Iroh?

"I don't have that luxury."