Some (read:most) is formulaic. Once in a while something is fresh. Writers are always looking for the angle to bring a new twist to the ancient story of boy meets girl. Like a sonnet, a few rules must be observed or your story/subplot does not qualify as a romance.
- The protag must be single at the beginning of the story. Possible variances include attached in the context of a bad/abusive/dying relationship.
- The couple must be together by the end of the story. Or, in a series, there must be clear intent to move in this general direction.
- During the course of the story, there must be tension. If our hero and heroine are the destined to be together/soul mates/love at first sight variety, the tension will be external, like a mutual enemy. Dean Koontz does this to great effect. The other end of the spectrum is the classic love/hate tension. They drive each other batty but have an undeniable chemistry and a moment of vulnerability somewhere during the story where they gain a better understanding of each other. Think Benedick and Beatrice in Much Ado About Nothing. I'm a sucker for this trope. And the du jour of pop fiction is the "I'm a human and my lover is not" variety. This one is usually resolved by "upgrading" the human in some way.
The beginning and end are pretty easy to master. They start off apart and end up together. The tricky part is how they get there, the journey and the tension. And this is where the shark jumping takes place.
Please. If you are writing a book with romantic elements, for the love of all that is holy, respect the limits of the ICK FACTOR. For the most part, the couple should not be so May/December that the reader is prompted to speculate about basic biological limitations like ED and reduced post-menopausal sex drive, for example. If you go there, you better be good. Harold and Maude? Soul mates, hilarious, and a great soundtrack worked to downplay the ick factor of a barely grown young man falling madly in love with an eighty year old free spirit. The Mary Russell books by Laurie King have a fifty something Sherlock Holmes fall for a twenty year old difficult genius. The age disparity is dwarfed by the complete lack of anyone else in the world who might be a suitable companion for either character.
Occasionally somebody pulls off a Thornbirds. I suspect this does not have the shock value it used to, though, so not many people go for the man-of-the-cloth trick to heighten tension.
But I read one over the weekend that is irredeemable. Absolutely not. NO WAY. Let me set the stage: YA UF. Usual cast of characters. Decent world building. Snappy dialogue. Love triangle (which is not my cuppa but I make allowances since it clearly makes millions swoon). So we have some nice little sparks flying in the middle of a generational conspiracy for genocide. Sweet little kiss scene followed up by the fellow being a complete jerk but we know he's not such a bad guy, simply conflicted. We're doing well on the tension front. But since the author has designs on a series (there are published sequels) there must be the Big. Bad. We can never be together. So what's it gonna be? Guilt? Angst? Misunderstanding of I Love Lucy proportions? No such luck.
INCEST. Incest, people. As in, the two people you are pulling for find out at the end that they are brother and sister. And yes, being a savvy reader I totally get that this will most likely be resolved in some convoluted plot twist involving a baby switch or something but the damage is done. These two people think they are siblings so there is no potential for anything but ICK. ICK. and more ICK every time they gaze longingly at one another, innocently brush hands reaching for the same death dealing supercharged silver tipped ninja star, whatever. It's not just no spark. It's anti-spark.
Greek mythology in UF? Totally acceptable. Greek tragedy? Not so much.