What makes two people compatible? Are there people we will always click with because we have some kind of innate chemistry with them? Or is chemistry a product of circumstances: two people meeting at just the right time in their lives or in just the right situation?

It’s a problem that romance novelists grapple with all the time. Though they create that chemistry for a living, even the most successful writers still struggle to pin down what exactly gives two characters their spark. “I’ll have this idea of, a person like this would fall in love with a person like this,” says Emily Henry, author of the bestselling Beach Read. “And then when I put them together, it’s just boring.”

For a recent episode of Vox’s Unexplainable podcast, reporters Brian Resnick and Meradith Hoddinott spoke with researchers investigating compatibility and chemistry, while I talked to romance writers to learn what romance novels can teach us about attraction. Not only do these books act as cultural artifacts, documenting what people yearn and burn for, they also leave room for drama and plot and tangled-up mess — unlike scientific papers.

“To me, feelings and compatibility are one area where it’s hard to be precise and scientific because humans are changing every single second,” explains Talia Hibbert, author of the hit Brown Sisters series. “I’m not the person I was even 10 minutes ago. That’s why science is tricky, and that’s where romance can fill in the gaps, because it’s all about the journey and the importance of changing together.”

You can listen to the episode here, and continue reading below for Henry, Hibbert, and other romance writing stars on their theories of what makes two people a good match on the page and in real life.

Talia Hibbert

Courtesy of Talia Hibbert; Bita Honarvar/Vox

For me, a lot of how I think about my characters’ compatibility stems from a book called Romancing the Beat by Gwen Hayes, which is about the structure of romance novels. She talks about characters who start off hole-hearted, as in with a hole in their hearts. Then over the course of this story, they become whole-hearted, as in complete. I love that so much, and I always think of it in terms of, I give each character something that they’re missing or something that is affecting them, that they need to overcome. I do make their emotional journey about them overcoming it themselves, but when I’m crafting their partner, I’m also thinking, “Who is going to support them in this journey?”

I feel like you could take a couple, wipe their minds, and then push them back together again, and if the other circumstances of their lives happened to make that coupling good for them, then they would get back together again. But equally, their lives could veer off in different directions and suddenly they wouldn’t be compatible anymore. Compatibility is a combination of the internal and the external because the external shapes the internal.

I think that everyone has multiple soulmates, and the older you get, the more time you spend in the world, the more time you spend with certain people, the number of people who could be your soulmate diminishes because your needs get more and more specific.

It’s probably not very romantic of me, but I don’t like the idea that relationships have to be super exciting. I know that there’s this idea in a lot of corners of society that your relationship has to be super fiery, or if you love each other too much and you’re too comfortable together, that’s bad. It’s really nice for relationships of all kinds, but especially romantic relationships, to just be very chill. To be good and nice, and it doesn’t change every time you come back to it. It just is what it is and you’re happy.

Casey McQuiston

Courtesy of Casey McQuiston; Bita Honarvar/Vox

If you’re looking for an answer about chemistry that’s grounded in reality, I’m the wrong person to ask because I am a hopeless romantic. Also I was raised just Catholic enough to be a little bit mystical, and I feel like things are meant to be sometimes, like the universe puts things in motion.

Real compatibility is about sustainability, about something long term. To me, love is comprised of three components: respect, admiration, and enjoyment. You can admire somebody, you can respect them, but they can be a total war to hang out with. It can be hard to fall in love with someone like that. When you have all of those things present, though, that is when you have chemistry and compatibility.

On a more abstract level, it’s when your partner’s favorite things about you are your own favorite things about yourself. You can’t have true compatibility without having a really accurate picture of each other and really knowing one another on that level. With my own partner, that was when I knew this was a really compatible pairing for me: The things that they liked most about me were my favorite things about myself. You just feel really seen.

Sarah MacLean

Courtesy of Sarah MacLean; Bita Honarvar/Vox

Love is born of — it’s so cheesy to say — you loving yourself and believing that this other person makes you and your life and your path better and more valuable in some way.

Love is about the promise of a future. It’s about the feelings in the moment, but it’s also about, “I want to feel this forever.” Great chemistry feels like you could never sleep again and just spend every waking moment with this person — and I don’t just say that about romantic love. We feel that way in life, about lots of people, dear friends and strangers.

You’ve seen this in film and in real life. There’s that kind of patter that happens when two people are just in a vibe together, and they’re just back and forth like a gunshot, right? It feels like it’s just escalating and escalating, and then, boom, it explodes.

Look, I’m a romance novelist. I know the money is in those beginning feelings. The truth is that when you love somebody for a decade, or three or four decades, it doesn’t feel like it did at the beginning. But the hope at the beginning is that in three or four decades, it will feel just as special, just as important.

Emily Henry

Courtesy of Emily Henry; Bita Honarvar/Vox

There are a lot of different kinds of compatibility, but with romantic compatibility, it’s all about what someone brings out of you. It is a very selfish way of looking at love, but I think it’s true.

When you’re reading about a couple, what really makes you root for them is seeing how each of them brings certain things out of the other. It grows the character. In real life, that’s something that’s really exciting even when you make a new friend, to feel like, “Oh, there’s this whole other part of me that I didn’t know existed, and now this person, for whatever reason, has the key to this little room in my heart.” It changes you. Compatibility is about this third space. If you have a Venn diagram, it’s that overlap between you and another person and how much you enjoy being there.

When writing romance, it all comes back to tropes for me. Those tropes exist because we see them time and again in real life, and so they get codified in books. In my second book, there’s the trope of best friends who fall in love. They’re also polar opposites: somebody who’s really high-energy and loud and kind of obnoxious, and somebody who’s very quiet and repressed. That’s a very valid form of compatibility that’s also really exciting, and it pulls on those characters’ boundaries and grows them.

It’s about building a character who needs to have someone who accepts them exactly as they are, or building a character who needs to have someone who can challenge them because they’ve fallen into a status that isn’t actually what they want. I’m starting with that trope and then I’m working back to make a person who needs that relationship.

That’s what we’re all looking for: someone we can grow and change with.