Zuko and Sokka growing close after the events of book 3 causes a whole other set of problems.
Zuko literally dreads night, or more specifically, SLEEPING, because it means he is completely alone in his Father’s old chambers, the Firelord’s chambers, and being back home was everything he ever wanted during his banishment? Right?
Except it wasn’t actually the palace he wanted back, it was his family. And now they’re all gone. And he’s the only one left to fix the mess they’ve left behind.
Every night, becomes torment. Because when he’s completely alone, in the dark, there really isn’t anything he can do to stop his thoughts from ripping him into shreds on the inside.
But when Sokka comes to visit, he is almost never alone. Because night, when the firelord is finally free from his daily duties, is the only time they can actually spend time together.
It starts with going out into the city, going on little quests in the woodlands not to far out, but eventually they lack of sleep catches up to them, and they’ll just stay in one of their rooms together and talk. Talking about things they couldn’t with anyone else, because no one else around them truly understood in the way they understood each other. And then eventually, one night, after so many sleepless ones, they both do finally succumb to sleep, side by side on Zuko’s bed.
It was the most peaceful rest he had had in years.
This routine becomes a problem in itself because Sokka can’t stay in the Fire Nation forever. And Zuko’s growing codependency on him is going to hurt him so much worse than the insomnia ever could.