Don't Look Down or Back
Katara knows Aang has been through so much; she was there at his side almost the entire time. He's gone through pain that no one his age, even as he gets older and wiser, should have to go through.
He had to take on an entire power-hungry nation with only a few friends and allies. He had to bear the loss of his entire culture, almost everyone he ever knew and loved in his past world, in silence. Katara says 'world' instead of life because, to her and him, it was an entirely other world. It wasn't just a different time, it was an entirely different situation and place. One that hadn't been torn and burned by war.
And Aang had to harbor all that grief and strife inside, had to hold it down until peace was restored and the weight of the world was a little less on his still-growing shoulders. After months of running for his life, he had been able to go back and grieve. To bury the bodies and place flowers on the numerous, lined graves.
Katara tries to help in every way she can, to heal him beyond her waterbending abilities. But there are some things she can't do. Yet there are some things she can prevent.
One of those is heartbreak.
She doesn't need any reinforcement for loving him, but at the thought of making him so lost and devastated by breaking up and leaving him behind- it makes her own eyes water. She can't imagine the pain it would put him. To have the first person he'd latched onto in this world leave him, too.
So when they argue, she always keeps in mind that they will always (always) make up. They won't leave each other; she can't leave him, he will not leave her. There's nothing infuriating enough that they can say to each other before sprinting back to the other's arms, lips meeting with their hands and hearts.
She doesn't want him to go through that pain, and neither does she. He saved the world, her world. In return she'll make a new one for him.
She will shower affection on him. She will soak him with love. Katara will positively drown him in enamor.
And Aang, in turn, will continued flying them up to the sky, weightless without grief.
AN: Just a thought that came to me when I thought of how Katara is Aang's first and last love :)