I'm guessing that if you're reading this, you want to write a romance novel that makes money. You'd like to sell more than three copies to friends and family.
How do you do that?
You have to write to reader expectations.
Rule 1: For a book to be considered romance, there must be a happily ever after or at least a happily ever after for now. (This is why Nicholas Sparks doesn't consider his books to be romance. Romeo and Juliet is not a romance. It is a tragedy.)
The hero and the heroine must end up together. (This can be over the course of several books if you have an ongoing series. I know Fifty Shades of Gray doesn't always end on a happy note, but the last book does.)
If you do not give your readers this, it is not a romance. Romance readers want the emotional payoff. That's why they picked up your book.
Rule 2: No cheating. The hero cannot cheat on his girlfriend to be with the heroine. He needs to break if off with 1st girl before dating heroine. (And vice versa) You can play with this, but know that cheating triggers a lot of unhappy reviews.
Rule 3: The romance is what is important. It is what the book is really about. There can be zombies and aliens and nuclear bombs, but if the book isn't focused on the relationship, then it's another genre with a romantic sub-plot.
Then within each sub-genre there are expectations. Sweet romance: no sex. Erotic romance: hot, steamy awesome sex. Historical has different rules than paranormal (historical peeps like factual details.)
How do you discover these rules? You read that genre. If you want to write historical fiction, you need to read a bunch of historical romance novels. Read the best selling ones and see what they do well. Read one that has terrible reviews and see why people hate it. (Or just read the reviews on those. They usually spell out why they didn't like it pretty clearly.)
Follow authors' blogs and Facebook pages. There are some great resources out there! (I'll be adding some links soon!)