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Connection
Toph didn't remember when she became an earthbender. There was no sudden epiphany or decision to become one. All she knew was that the earth spoke to her. Every time her bare feet touched the ground, in its own rumbling voice the earth called to her. When she asked it where her parents were, vibrations traveling up her feet carved the answer into her mind, and she knew she could see. When she asked it to lift her higher, it raised her a platform and suddenly, she was king of the world.
Katara didn't remember when she became a waterbender. Growing up in the water tribe, she remembered always relating herself to the healers and warriors, to the giant waves of ice cold blue flashing back and forth and suddenly disappearing. There was no moment where she wished to become one. All she knew was that every time she touched the cool liquid, its murmur invaded her senses, and images of fish or icebergs passed through. And when she asked it to writhe and twist in battle, it knocked unseen arrows off their path to her heart
Aang didn't know when he became an airbender. Feeling the wind brush upon his skin on a hot, summer day, he had always always grateful to it. All he knew was that the air always whispered to him, carrying news of a band of thieves or traders and imprinting the sounds in his head. When he decided that he wanted to fly, the wind picked him up and carried him away.
Years later, when he suddenly understood that the water murmured to him to swim far, far away, when he suddenly understood the earth rumbling and reassuring him that it would be sturdy, when he suddenly understood the fire hissing at him to attack from the left, he knew that he was truly the Avatar.
Zuko remembered when he became a firebender. Being Fire Nation royalty and seeing his family blow embers to life, he made a conscious decision to become one. He remembered being burned and singed, struggling to tame the flame. It never flowed, but jerked and pulled from him, refusing to bow to the boy.
It was never natural for him to find the most flammable areas to bend in when fighting the Avatar, nor was it ingrained in his soul to shied from an abundance of water.
And it wasn't until Toph gave that delicate little secretive smile of hers as she dug her bare feet into the soil outside of their mansion in Earth Country, that he realized the hiss of the fire was nothing more than a misunderstood language to him. He was never a master, nor would he ever be. But the ring of his broadswords as they cut through the air, the easy movement and connection he felt as he swung at an enemy, had always been alive.
So no, he would never master fire. He just wasn't a bender. But his broadswords were his, and his alone.