Disclaiming Banner: Let it be known that while I do hold a touch for Zuko, I do not hold the copyright for ATLA.
Sappy
The reason Sokka had disliked Zuko for so long, was mostly because of Katara. The chasing and blowing up of stuff had a lot to do with it, but yeah...Katara. His sister had the worst taste in men. And here was an admittedly good looking, potentially wealthy, healthy teenaged-boy; guy.
Now he wasn't much on the Protect-From-Everything front. Katara never would have left the igloo much less the South Pole if that were the case. But he could try to be the Big Brother about some stuff. Fortunately, the jerk-bender tried to avoid everybody who wasn't Aang or Toph for the just the longest time. By the time Sokka got it in his head to go looking for Dad, he had started to wonder if Zuko was the asexual, fire-bending, honor machine that he'd always kinda dismissed to the back of his mind before.
Except...little details like the portrait of his Uncle, or letting The Duke tell him stories about life in the forest. He was cushiony around the edges now. He even brewed a mean cup of tea. Also, he still didn't seem to have anything below the belt.
He understood the situation completely when charcoal-breath talked about Mai. Sokka knew that goofy tone of voice pretty well.