Im a zutara shipper, and i know this is a little bit of a stretch but I noticed something cute while I was rewatching season 2! When Iroh is teaching Zuko about redirecting lightning, and tell him how he learned the technique by studying water benders, zuko gets this little look of surprise and then smiles slightly. The man was thinking about Katara like plsssssss I can’t. Even if it was more of a determination or smug thing still it’s so sweet
So you know I get my zutara crumbs when I can, but I think Zuko's expression doesn't really have to do with Katara. At least, not in a romantic sense. But thematically I do think you can connect it to their relationship
When Iroh tells Zuko he learned the technique by studying waterbenders, Zuko looks initially surprised, because he has a hard time accepting what his uncle is describing, what he says is "Avatar stuff." Things he can't imagine needing to learn because it's so different from what he's been taught is important, fed on a steady diet of Fire Nation superiority. The idea that Iroh could become a better firebender, could teach Zuko how to be a better firebender, by studying waterbenders, is so counter to that. It's also very close to treason, because the idea that we should learn from others threatens everything the Fire Nation has built their empire on.
But Zuko accepts it because Iroh tells him it's something not even Azula knows. He's not yet at the understanding that he needs to learn from others, but he's realizing that Iroh is teaching him something that will give him an advantage over Azula, who would never think to learn from others. That's why Zuko's expression becomes determined.
It's not all that much of a stretch to think he also thought of Katara, because she is the waterbender he has fought before, and she beat him. As hesitant as he might be to learn from enemy nations, he knows that waterbenders are powerful, and identified Katara as a powerful threat and his opposite. He also starts learning from Katara and the others, and his uncle's lesson about learning from the other nations probably had a hand in that.
And, of course, the lightning redirection will be key in the final battle with Katara. So while he is not necessarily smiling because of Katara, he is smiling at the realization that he now has a tool he can use, and he's subconsciously learning from Katara even while she is his enemy, and learns a technique that he will eventually use to save her life.
And, of course, thematically, Zutara is all about learning from others, and is directly contrasted with what Azula represents, particularly to Zuko, which is what he could become if he never learned from others, if he never let go of his pride or atoned for his actions, if he never learned to care about others.
Water is the element of change, after all.
