Romance requires obstacles. Any good plot does. In romance, you need to have a pair of people who desire to be together, and--more importantly--you need strong reasons why they can't or shouldn't be together. That's your plot, your conflict.
In the old days, this was easy. Obstacles to romance were everywhere. For instance:
Class/race divide: A governess can't marry the master of the house. An heiress can't marry the chauffeur. An Untouchable can't marry a Brahmin. A white woman can't marry a black man. Etc.
Arranged marriage/ No divorce allowed: Turns out you married a jerk? Too bad! You're stuck! (Until you get lucky and he dies, which will happen about 90% of the way into the book, so hang in there.)
Long-standing family feud / Being on opposite sides during a war: ...Self-explanatory.
Technically, any of the above problems could still take place today, given the right culture and situation, but they all feel pretty outdated. This is why, to take a small tangent, historical romance is also hot, and always has been, and always will be. Obstacles to happiness in historicals are easy to find. They're littering the field! Good luck not running into them! Similarly, a dystopian future can have any such obstacles you want to create, and those stories are fairly popular too.
But in contemporary romance--you two want to be together? Go ahead. Couple up. These days, you can divorce a jerk. You can move away from your parents if they're a pain about your choice of mate. You're protected by a lot of shiny new laws ensuring your pursuit of happiness. And that's great! For humankind, I mean. It sucks for romance writers. We can, and do, explore the remaining taboos and tangles that snarl up couples, but those are trickier. Love triangles tend to be messy if realistic, and annoying if unrealistic (why wouldn't the triangle quickly resolve into a couple if that third person isn't likable?). Geographic distance has its possibilities, but again, it's usually not hard to overcome in modern times. Taboos like teacher/student, doctor/patient, and boss/employee can work, but also run the risk of squicking people.
(Yeah, I wrote about teacher/student anyway, and yeah, I think it did squick some people, despite the characters being well over the age of consent. I wouldn't have dared write a story where one was under the age of consent. Nabokov's braver than me there.)
Oh! But! You know what would be a seriously great obstacle? What if one lover was human and the other was a vampire? Or a werewolf? Or a ghost? Or an angel? Or a selkie? Or a faery? Or a--yeah, you get the picture.
And it's true. These are not only interesting, fanciful story ideas, which bring lots of exciting mortal peril (also vital to a romance tale), but they come inherently packed with the crucial obstacle to happily-ever-after. So that, my friends, is my big and surely non-original theory on why the paranormals are selling like shape-shifting hotcakes lately.
That and escapism. Reading is always about escapism, and elements of fantasy increase the escape velocity (so to speak). But you knew that.