Luna didn't fall for Harry at first sight.
She did notice right away that there was something special about him, even beyond being The Boy Who Lived. But after all, he was with the girl who called her Loony. And though Luna tried not to let the nickname bother her — after all, she was a bit spacey, and she saw nothing wrong with it — she knew that there was always at least a bit of condescension. So when no one spoke up to correct Hermione, Luna was unsurprised but nonetheless a bit hurt.
It was when they were with the Thestrals that she first realized how she felt. She was comfortable with him, and loved him as her friend: that much she already knew. But when they were with the magical horses, she came to know a twinkling of something else within her. But she could already see how he felt about Ginny. She didn't want to press him, nor risk her friendship with either.
But more than that, she didn't want to be in love with Harry Potter. So she held it back, as best she could. She tried not to think about it. And eventually she found that she could hardly stop.
So it hurt all the more when she learned that he'd kissed Ginny. Even after all that time, a part of her had hoped nothing would come of his feelings for the redhaired girl. But the kinder part of her was genuinely pleased for her friend. I want him to be happy, she thought. So why am I so upset?
Ironically, Hermione — the very Hermione who'd unintentionally mocked her that day as they went up to Hogwarts — was the person whom she turned to for help.
"That's what it feels like sometimes when you fancy someone, Luna," Hermione said. "It hurts a little, doesn't it?"
Luna nodded slowly. "But it feels so nice, too."
Hermione smiled, a bit sadly. "I know. I know."
"How do I get past it?" Luna murmured, mostly to herself.
Hermione tilted her head. "I think you just have to keep trying your best each day. To be a good friend, and to love the people close to you, even though it hurts."
Luna smiled, her slightly daffy half-smile. "Thank you, Hermione. I know I must sound like I'm suffering from an attack by Nargles."
Hermione tried not to bristle at Luna's simple acceptance of the mythical creatures' existence. "Not really, Luna," she said honestly. "I have a pretty good idea of how it feels."