Never
Never in his life Aang saw his friend Zuko being so happy.
The Avatar was surprised because after traditional greetings Zuko's first words were: "I killed them."
Zuko killed somebody? Aang couldn't believe it and asked for a little more detailes.
And Zuko explained.
Well, six months ago Zuko had a very big failure. The most terrible failure in the whole his life. He put his little sister, Azula, in an asylum. That was a good step but the asylum was wrong.
Shortly speaking: doctors were sadists.
Well, 6 months Azula had nothing good. As soon as Zuko was informed that his little sister had been raped by the guys, who were supposed to cure her, he visited the asylum.
Fire Lord killed the doctors. All of them.
"You know, Aang," Zuko said, "I saw that you were wrong. Life is not a sacred thing. Sometimes a living person must be killed."
So, Fire Lord punished the doctors but it was to late - Azula was hurt by them. Fire Lord didn't protect his little sister.
When Zuko brought her in the palace he asked her to forgive him. He didn't mean to put her in that scary asylum. He didn't mean her being hurt those 6 months. He didn't mean...
"I will," Azula interrupted him "never forgive you. NEVER."
...
Fire Lord finished his story and now Aang was very very very very VERY surprised.
"Zu... Zu... Zuko... WHY, for heaven's sake are you so happy?"
"Aang, don't you get it? I need Azula in my life and she said she would never forgive me."
"Yeah... so..."
"She. Said. She. Would. Never. Forgive. Me. Never. Aang, NEVER. But I know it: AZULA ALWAYS LIES. So, I have a chance to be forgiven by her. That is why I am so happy."
Thanks for reading.