Do you mean, do we need to love as we are writing, or do we need to have experienced love in order to write about it? Also, what kind of book? A how-to on relationships? A romance novel? A fantasy with a romantic theme?
Maybe. But we don't need to have done everything that's included in our stories - I think that takes 'write what you know' a bit too literally. I can write about space ships despite having never been in space. So ... maybe. But I'd argue probably not. Romantic love, especially, is such a prominent topic in books, poems, tv shows, movies, songs - in just about everything - that I'd reckon you can write an approximation of it just by being exposed to everyone else's portrayals of it.
You are limited only by your imagination. If you can imagine what it feels like, you can write about it.
