He couldn't understand it.
She was so simple, yet so complicated. Their laughter bounced right off her. She stood out and wasn't afraid.
She was luminescent. A bright sort of energy radiated off of her, and he felt like a rabbit caught in the path of a patronus.
He wanted to be closer to her, to absorb her indifference. He wanted to give her turnips and rescue her belongings and throw pumpkin juice at anyone who called her 'Loony.'
He wondered why she wasn't in Gryffindor; she was so brave! Who else would wear unicorn earrings and two different pairs of shoes and tuck their wand behind their ear? Why wasn't he in Hufflepuff?
He was clumsy and dopey and awkward, but she was graceful and beautiful and brilliant! She was everything he wasn't. She was…Luna Lovegood, goddess.
And when Dumbledore died, she was there. When he needed someone who wouldn't laugh at him, she was there.
When she found him behind the greenhouses crying, she didn't make fun of him. She took his hand in hers and just danced with him. No music, no reason: just dancing.
He decided right then and there that he loved her.