To me it really looked like Zuko cared more about Sokka and Katara than Aang did, just look at how happy he was when him and Sokka brought Hakoda back, he was so happy for Sokka and Katara to see them have their father back, it never showed Aang be this happy, and the way he acted for Katara wanting to fix her pain of losing her mother is just phenomenal, Aang never asks Katara about her mother, he never asks how she feels ever, he always just looks at her as the girl he's wants to be with...
Yeah, from the very first time they talked, Zuko and Katara could connect on the issue of their parents in a way she never could with Aang. It is very clear this was meant to be a major point of connection for them. Katara opened up about her mother, and Zuko talked about his scar, which his father gave him.
He was meeting Ozai again just when Katara was meeting Hakoda again. What is Aang doing during this time? Running away leaving her feeling abandoned.
Zuko and Katara both struggle with missing their mothers. Zuko cannot talk to any of the girls about this, especially his girlfriend. And Katara never talks to her friends about this. Aang has no idea how she felt, he spent the episode gambling and playing.
Zuko goes back to Iroh, his real father but he’s gone. Aang Katara has to separate from her dad. Aang never asks about Hakoda after this episode, but Zuko is the one who brings him back.
And Zuko is there when Katara faces her mom’s killer. And Katara is the one who is there when Zuko reunites with his uncle During the whole finale he is missing because he ran away. Aang never connects with Katara on a deeper level. I can see a lot of evidence that Zutara was originally planned, but sabotaged.