Romance is a lot like chocolate

Your palms are sweating, your heart is racing, you’re tongue-tied and you’re certain that whatever you do, this situation is only going to get worse.
The brief scene described above could be from two very different stories.
THE FIRST: For the past two days, you’ve been tracked by a serial killer. You finally make it into a Mini-Mart and you want to tell someone what’s going on, you want to ask for help, but you’re afraid they won’t believe you.
THE SECOND: You think you’re falling in love.
As strange as it may seem, many of the symptoms of falling in love are similar to those experienced during moments of stress. Queasy stomach, inability to sleep and lack of concentration. Is it any wonder then that romance often finds itself frequently mixed in with other, seemingly incongruent, genres?
Why does this technique of genre blending work so well? Probably because it gives the author two distinct plot lines. Whenever one subplot feels like it’s cooling down, the other subplot begins to heat up. By switching from one plot to the other, the author is able to keep the reader on the edge of her seat, wondering if a) the heroine is going to be eaten by zombies or if b) the heroine is ever going to realize that the hot boy she’s hanging out with is really in love with her, and she feels the same way.
Another reason blending romance with another genre works so well is the fact that falling in love and fleeing for your life are both adrenaline-packed actions that are fraught with danger. Ever tell someone you’re crushing on them only to have them answer, “I don’t feel the same way.” Life changing. You instantly wish you had a cloak of invisibility.
When it comes to storytelling, romance is a lot like chocolate. It goes with everything. It goes with mystery, horror, science fiction, fantasy, action adventure and literary fiction. It not only goes with these genres, it enhances them. It makes them stronger, gives them deeper themes, greater stakes, higher rewards.
In other words, the addition of romance almost always makes stories better.
Some popular YA examples of genre blending:
Mystery/romance: The Body Finder by Kimberly Derting
Horror/romance: Ashes by Ilsa J. Bick
Sci-Fi/romance: Enclave by Ann Aguirre
Fantasy/romance: The Hunger Games by Suzanne Collins
Action adventure/romance: A Great and Terrible Beauty by Libba Bray
Literary/romance: The Sky is Everywhere by Jandy Nelson