As I mentioned before, I've been writing fanfiction as a refresher on writing in general, and most fanfiction is romantic in nature: OTP (One True Pairs), ships and crack ships are the catnip of the fanfiction writer. And as I've been writing them, I've come to appreciate something a romance writer told me a long time ago: every romance is a threesome, and the antagonist isn't who you think it is.
Let's review the parts of a story: A character is someone with a goal, motivation for seeking that goal, and conflicts between herself and that goal. The protagonist is someone for whom the external goal arises suddenly, a threat is introduced and starts the story. The protagonist is someone with a problem. A scene is where a main character shows up and attempts to move toward his goal; that is the reason he is in the scene and the reason the scene exists. He will fail in some way; his failure sets up a future scene. The sum of all such scenes makes up the plot.
In a romance, "romance" is the last thing on the main characters' minds when they're introduced to one another. They may have antipathy, antagonism, lust, avarice, greed, or some other goal they want satisfied in the course of their introduction to one another; each may simply want nothing to do with the other. Your goal as a writer is to introduce something that, through the course of your story, brings each to understand that the other person is the best thing that could happen to him or her.
That something is the relationship. Remember: each protagonist has preferably two goals at the beginning: an external one (marry a prince) and an internal one (not feel weak in front of one's peers). That's four goals, none of which should, at the beginning, suggest that these two characters belong together.
So what draws them together? The relationship. The relationship is an antagonist, and you should write out its goals, motivation, and conflict. For example: Goals: "Get these two characters together / resolve the tensions between them"; Motivation: "the relationship will blossom / the relationship will last"; Conflict: "he's seeking someone of noble birth / she's just coming off a bad relationship and has eschewed all men."
In every scene with either character, the relationship is there. Ask yourself: how does it sneak up on him or her? What does it make each say to the other that furthers its goals? How does it power play the two of them against each other in dialog, furthering its interest in their best interests?
Thinking about the relationship this way, as something each character will seek to avoid or undermine in her own way, can make romance writing a much more entertaining and viable.