When it comes to creating dramatic tension, I've talked about creating characters readers can care about, conflict, misunderstandings, complications and expanding the stakes from the personal to the public. What other ways can you raise the stakes for your characters? After all, too many quarrels can sound like petty bickering; too many misunderstandings that aren't resolved can make your characters seem immature. You can only put your characters in physical danger so many times before your story starts to sound like The Perils of Pauline.
Consider, then, something that can be either good, or bad, but that will certainly affect your characters' relationships, and thus your story: physical intimacy and emotional intimacy.
Think about stories where people are forced to be together, either because of a natural catastrophe or man-made situation -- a prison drama, for instance, or a "buddy" picture. In a romance, think stranded in a cabin during a blizzard, or an arranged marriage. The fact that your characters are, in one way or another, trapped together, adds to the tension and affects their relationship.
There's another kind of physical intimacy that, as a romance writer, I'm naturally going to be thinking about, and that's a sexual relationship. That sort of intimacy has to change a relationship, for good or ill. How much would depend on the characters and their situation, but if it doesn't change it at all, why bother writing about it? They might as well be brushing their teeth.
There's another kind of intimacy that can really up the tension in any novel, but especially in romances: emotional intimacy. Everybody fears rejection, so getting close enough to another person to reveal and share one's innermost feelings, can seem very risky indeed. What if you do that and the other person rejects you? Many of my characters would truly rather face physical danger than run that risk. Even the very thought of being emotionally intimate raises the stakes enormously for those characters.
Dramatic tension doesn't have to come from a big explosion, a natural disaster, shots in the dark, a huge quarrel or some deep-seated trauma. It can exist in the possibility of fingertips touching, or wondering if anything should be said. Who hasn't been reading a book, watching a movie or TV show and thought, as two characters are together, "Oh, please, say you're in love and kiss!"
That is dramatic tension, too.