Things start to go bad for Asami and her father Hiroshi's relationship early on in this season of Avatar. Before that, though, they had a great relationship -- they're much of what each other had, after Asami's mother was killed by the Agni Kai Triad during a robbery. Asami was six when she lost her mother, so Hiroshi raised her. It's obvious that losing his wife and his child's mother would affect Hiroshi; his response was to join the Equalist movement.
He kept it a secret from Asami, especially after she met Korra. But when they found out, it was worse than they thought. He'd also been manufacturing equipment for the Equalist army. Using mech suits, Hiroshi and other Equalists captured the very people his daughter Asami had been working with. No, Hiroshi! Later, Equalists even captured Asami. Hiroshi, still, just used this attempt to get his daughter to come over to the dark side -- to join him in hating benders. When they escaped, HE ATTACKED HER. He was going to kill her. Hiroshi was going to kill his own daughter. Again: what the fuck?
When he's finally captured, he "repents" in prison. He tries to have a relationship again with his daughter that he was trying to murder. Asami, obviously, was super hurt by this. I mean, devastated. Asami hadn't forgiven him, but he tried, which makes this episode hurt even more.
During the epic battle in the episode "Day of the Colossus," Hiroshi sacrifices himself to save Asami and her cause. Asami and Hiroshi were working together to pilot a mech suit, trying to take down an ever bigger one. It was just like the good old days. They'd basically just made up, when Hiroshi realizes that he needs to sacrifice himself and eject Asami from the suit. If he didn't do that, the mission might have failed.
Just as Asami felt like she has her father back, she loses him. Why do you keep doing this to me? Why do I want new Avatar series if it means I'll feel the pain all over again?