Being ceremony partners for years is not enough for the time you spend together. For your father, you and Zatanna are almost brothers, for her, it is completely different. You two have homo-magi, so you practically grew up together since you were little, learning about magic while going with her father to the magic shows that Giovanni did.
Over time, you and Zatara grew up, becoming ceremony partners in small magic shows, whether for children who wanted to see an ugly clown, or in seedy bars, where drunks abound in misfortune. Either way, you and Zatanna stayed together, and she was becoming more and more sure that she liked you, very romantically.
While you were trying out some spells for a child's birthday party, Zatanna just watched you, like a schoolgirl looking at her crush, only this time she's 23 years old. Seeing you so focused on your magic is cute and his heart is beating rapidly in proof of it. Zatanna has done everything to make you realize the reality, with hints so obvious that anyone would notice.
You are different. You didn't understand anything.
"Can I try one?" Zatanna suggested, taking the spell book from you. She turned to page 116 for a spell that normally wouldn't be used in magic shows for small children. A love spell. It's the most extreme thing Zatanna has ever done.
But if it's for her crush, for you, she would do anything.
With a few backwards words that you only understood three of—longing, charm, and desires—she activated the spell, shooting it straight towards you. A somewhat manic smile escaped Zatanna as she saw you looking confused and disoriented, while your eyes turned into a heart shape.
"Oops! My bad...", she exclaimed, with mock guilt before adding a "Yay!" in a low voice when the spell worked. Or so she thought at first glance.