The first thing you should note about dating a Harry Potter fan is that her obsession with the series is completely justifiable. In addition to being the greatest magical boarding school epic of all time, Harry Potter holds a great deal of sentimental value for those who grew up with him. When the first Harry Potter book came out, I was 11, just like the titular character. Up until my senior year of high school, Harry and I were roughly the same age. I might not have grown up as an orphan or dueled with a tyrannical wizard, but Harry suffered with me through classes, detention, and botched teenage relationships. To know Harry Potter is to know the child inside, and the more you can appreciate a fangirl's enthusiasm, the more she'll appreciate your own eccentricities.
By indulging a Harry Potter fan, however, you risk becoming one yourself. Not that this is a bad thing when it comes to relationships. Once you're all caught up on the stories and the spells, you'll realize that Harry Potter isn't about magic at all, but about something much more integral to a meaningful life: sacrificial love. While adults frequently snub the books for their ostensibly childish motifs, anyone who has ever read them will know that the enchantments are merely vehicles for a larger story about the triumph of good over evil.
Despite its mature themes, Harry Potter will always attract mild ridicule from people who don't understand how the series can lure adults into a world created for children. Mock it all you want, but this fandom is a breeding ground for whimsical romances. There's no return from a love affair with Harry Potter. Instead of attending wine tastings, the two of you will spend hours concocting the perfect Butterbeer recipe. You will Sort each other's friends into their respective Houses. You will guess each other's Boggarts and speculate endlessly on your Patronuses. You will grow to appreciate the resemblance between wizard photos and animated GIFs. You will also have at least one argument over whether her mother is a Slytherpuff or pure Slytherin. But mostly, you will wish that J.K. Rowling would write another book.