I think a lot about how Sokka would react to the origin of Zuko's scar
Zuko would talk about it with Sokka mainly because he trusts him, and thinks that the two of them have a lot in common, so they understand each other very well, so Sokka ends up being the first to find out how the prince got the scar
"He did that?" Sokka was perplexed, how could someone do such a cruel thing to his own son?
And Zuko just laughed nervously: "You're the first one I've told, not even Aang knows."
"I simply don't have the words to express all the hatred I'm feeling for your father right now, of course he's the monster who tried to wipe out everything I knew... My family and my friends... But this? That's unthinkable!"
And the situation doesn't improve when Zuko reveals why his father did this to him.
"HE DID THIS SIMPLY BECAUSE YOU 'SPOKE OUT OF TURN'...? He banished his fucking son simply because you HAD PASSION AND CONSIDERATION FOR YOUR PEOPLE! Quite unlike him, I must say..." Honestly, it had been a long time since Zuko had seen Sokka so angry; in fact, Zuko didn't know if he had ever seen the water tribe warrior so angry before.
After that day, their relationship changed. Surprisingly for Zuko, Sokka never seemed to treat him differently because of this, although he also knew that Zuko couldn't hear or see properly on the left side of his face. Sokka didn't treat him as if he was fragile or needed excessive care because of this.
And Sokka had no idea how important this was to Zuko.