He hadn't paid any attention to the Ball his mother was planning. He spent his days sulking about the manor, or sitting in uncomfortable armchairs attempting to read books he had no interest in. His thoughts were always on Luna, wondering where she was and what she was doing at every moment of the day. And at night, he lay awake, unable to sleep for the painful sense of longing that he felt for her. He knew that on some level he loved her because she represented his childhood, his lost innocence. But he also loved her, the way her wavy hair caught the sunlight and turned gold, or the moonlight and turned silver. The way her skin had looked against the faded pink velvet of the bench. The feel of her fingers tangled in his hair. The quietly neutral look that graced her face when he fled.
And as he thought of her more and more, he became more and more uncomfortable with the idea of her. He wasn't just nostalgic for his lost childhood, he was bitter. And he was ashamed of the things he had done as a child, when she was playing in the sunshine outside her odd tower of a house. While she had been making daisy chains and naming clouds, he had been diligently learning from his father, learning things that no boy of his age should know.
His anger that had previously been directed inwards at himself began to be directed at her, for having such an unsullied life. He was so indescribably jealous of her, of her loving parents, of her ethereality. Why hadn't that been him?
He knew it was sick, and wrong. But trapped in that enormous house, with only an army of house-elves, an easily distracted mother, and all of his pain, her couldn't help hating her.