One half of your couple makes the other half jealous.
If Luna really loved him, Draco thought, she wouldn't spend so much of her time away from him, searching for wrackspurts.
She was always buried in some thick and not very credible book, searching through it for descriptions of mythical creatures, or out on expeditions where she might stay out of the house and in the woods for days on end, and she was very rarely at home, with Draco.
When he told her as much, she laughed.
"Why, Draco," she said, "you aren't jealous of the wrackspurts, are you?"
It sounded rather silly when she put it that way.