Personal alliances can be critical to the success of a story. To me these include acquaintances, friendships, close friendships and romances. I find that it's really important, if I'm going to have two characters who have a friendship, to understand why it is that they are friends. One of the potential pitfalls of writing stories is that we can set up our cast of characters and place them on stage, announcing, "these people are friends" without deeply considering what they have in common and what brought them together. When we have two people fall in love, this can (I suppose) happen instantly and simply through pure physical attraction, but I like to consider a little more than just the "hhhot" factor when trying to get two people together. Indeed, I find it even more fun when the people are coming together in a more complicated way, as through adversity, overcoming dislike, becoming attracted without realizing it, etc.
I'm going to consider some basic alignment ingredients first, and then talk about engaging in the process of bringing two people together for an unlikely romance.
My current protagonist, Tagret, has three friends he always hangs around with. They have ended up together partly because they're all well-bred boys with a lot of money, a good deal of kindness, and an interest in school. However, each of them has an additional reason to hang around Tagret, and that affects how they deal with him. Gowan is very politically oriented, and not only does he like Tagret, but he recognizes the strength of Tagret's political position (through his father) and thinks it's advantageous to hang out with him because of that. This means the other boys are inclined to ask him for advice on political matters. Fernar is strong and everyone wants him on his side in a fight, but he feels less confident politically and values Tagret and Gowan for what they can teach him. Tagret's best friend is Reyn. Why? Because both of them share the experience of living alone with a sibling and a house full of servants while their parents have been sent to other cities for jobs. Because of this, they stick together and help each other through trouble more than the others. Details like these not only make it plausible that these boys would be friends, but allow each person to interact differently with the others.
We can think of these as things that the characters have in common. We can also bring characters closer in a story through letting them face adversity together. Events that bring characters closer are generally increasing the ways in which the two people align with one another.
Okay, so let's look at romance for a minute. The #1-A thing that everyone is going to think of to bring two people together is physical attraction. Yep, no surprise there. But particularly if a romance is going to be building over the course of an entire story, and ending in a lasting relationship rather than just a one-night stand, there has to be more to it than just hotness. I will tend to think of it in terms of two lists: 1. the list of things that separate the two people and 2. the list of things that they have in common. If I cause events to negate the effects of anything in list 2, the people will fall apart. If they are going instead to form a lasting relationship, any extremely serious objections or separation elements from list one have to be directly and deliberately countered by adding elements to list two.
In my Varin world, caste level distinctions are huge - cultural as well as legal - and not easily countered. One of the cross-caste relationships in my books results because a servant sees someone else disguised as another servant, and becomes both physically attracted to her and intellectually engaged with her before he realizes his mistake. Importantly, thought, he is also at the same time learning things about the historical origins of the caste distinctions that cause him to doubt what he'd always believed about them. Another relationship begins more subtly, because one of the characters feels so intimidated by the physicality of the other that his physicality impresses her more than his caste. Here the intimidation itself is an influence that keeps them apart - however, because it causes her to disregard his caste, I can then work on removing the sense of intimidation, and thereafter the caste factors will have less power to separate them.
I keep the alignment/separation lists in my mind when I'm writing conversations, and when I'm writing descriptions. If one character expresses unadulterated admiration for another, then that is going to suggest bringing them together - but it may not be realistic or appropriate for that person to express such admiration. I therefore play with ambivalence by putting expressions of admiration and separation in the same sentence or internalized description. "Alien but beautiful" might express one kind of ambivalence. "Possessing refined warmth" also suggests a contrast that might be meaningful to a different person.
Whenever we work with close relationships, we end up playing right along the edge of discomfort. Getting closer with someone else involves considerable risk. This should be reflected in the writing. Often events that tighten alignment also cause discomfort or a sense of invasion. Then the question becomes what to make of that discomfort - whether to soothe it, or to intensify it, etc. Discomfort is an opportunity for a writer who is trying to align two people, because situations that are uncomfortable are often perfect for initiating change in a person's mind.
When you're working with relationships in your stories, do take the time to ask yourself some questions like those below:
1. Why are these people friends? What specific things do they find most compelling about one another, and why?
2. Does this relationship require a backstory of specific aligning events, such as hardship? Or does it simply require basic common conditions?
3. How big a role does physical attraction play in the relationship between these characters?
4. What keeps or pulls these two people apart?
5. What ingredients might be able to counter the factors listed in question 4, and bring them together?
6. Can I use common experiences to erase the effects of prejudgment?
7. Can I align these two people in a similar way relative to a third party, event or task?
8. Are these people aware that they are coming together? If they are, how do they react to the knowledge? If they aren't, why aren't they? Is there something about the nature of the separation factors that keeps them from considering the possibility of their attraction?
Please be aware (though it should be no surprise to most of you) that I'm coming at these questions from a human-relations and anthropology viewpoint rather than a romance-writing viewpoint, so I can't speak to the particular requirements of the romance genre. However, I hope that these considerations can help you in thinking about relationships in your own work.