Beginning
He thought she was just another one of Bolin's fangirls- to be discarded like a piece of trash the next day by a very satisfied earthbender.
Then, she was the teammate who screwed up the game, but later came back from the danger zone (literally) to save both him and his brother, who, God knows, had no chance of winning on their own.
After she helped save Bolin from Amon, he started thinking of her as something more- not just a teammate, but a friend.
He found Asami, though, and she was quickly dumped from his mind. It was now the raven-tressed, sophisticated beauty who occupied his every thought, not the reckless, tomboy-ish Avatar.
When she kissed him, every thought of Asami was expelled from his head, and all he could was how it felt so right.
Then the arena was attacked, and she went from thought-consuming, frustrating, stunning teammate/Avatar/friend to someone who very much needed a hug. But she also became his hero, for risking her life to protect the very people who had just cheated her out of the championship.
When he threatened her with their friendship, she fell off of her pedestal in his mind. She was harassing his girlfriend, and he knew it was out of jealousy.
(Maybe it wasn't, he decided, after she told him Asami needed him more, offered them all rooms at Air Temple Island, forgave him for blackmailing her, and was right about Hiroshi. She got her pedestal back.)
When she was kidnapped, he lost his mind with worry, and thought, if he could never see the beautiful, wild, strong girl again, he might just die.
Then, she came back, and everything was right in the world. But he noticed something in his feelings towards Asami, something that felt wrong.
Her bending was lost, but she still kept her elevated place in his mind. Even without her bending, she was the most loyal, brave, selfless person he had ever known, and he couldn't imagine his life without her.
She got her bending back, and when he saw her in midair, eyes glowing, manipulating the elements she so perfectly embodied, he knew that he loved her.
(He had already told her, but every time he saw her, his heart confirmed it yet again.)
When she says I love you, too, he knows that the reckless, tomboy-ish, thought-consuming, frustrating, stunning, jealous (not), beautiful, wild, strong, loyal, brave, selfless fangirl (not)/teammate/friend/Avatar/hero/girl/person/love was perfect for him from the beginning, even if he couldn't always see it.