Little things
When writing romance it's important to make sure it's realistic. Thane recently wrote about something similar in her post Romance: Please write it right or not at all. Instead of the overview that she gives you I would like to go into some specifics.
When you love someone, or when you're falling in love it's the little things that make you love them more everyday. Maybe it's the way she can't help but mouth the words to every song she knows, or the way his smile reaches his eyes whenever he's cooking. Maybe it's the way he looks at her when she doesn't notice, or the way she always checks her phone to see if he texted.
Writing romance isn't just about lust. It's about becoming friends, learning about the little things. His favorite movie, her favorite drink, what his nightmares are about, and what her daydreams are made of. Love is a word that had become overused in our culture. So much so that there are times we mistake lust for love.
Like Thane says, your characters have to be friends before they can fall in love. And even after that they need to continue to be friends.
The beginning
Their relationship begins when they meet. Wether thats when they were young, when they started working together, or maybe when they passed each other in the park. It might have been a look, or something they said. The heart is a tricky thing, and it doesn't always go the way you expect. Maybe he's dating a hot rising euntrirpronuer who he's always thought he's helplessly in love with, but ends up falling for the girl who owns the flower shop down the street. Simply for the reason that he can't figure her out. She doesn't believe the world owes her a thing, she's a genius but hates for people to know it, and she loves cats. Those are her little things. His are that he likes to brew his own coffee, he's terrified of dogs, and loves sappy romance movies. They start traditions, and inside jokes, they're able to laugh together, and something happened where they saw each other at their most vulnerable. Maybe she's frightened of thunderstorms because she's had bad experiences with loud noises and he's there to calm her down. Or maybe he suffers from PTSD after a traumatizing car wreck that nearly killed him and so she offers to walk to the store instead. Thats the beginning, the seed that burrows into their hearts.
The blossom
When that seed begins to germinate and bloom there will be subtle changes in their behavior. Maybe they turn down a day out with friends to watch a movie with each other. Or maybe there's a present that no one else payed enough attention to get. A tune they always sing together stuck in their head, and they're not sure if it actually bothers them. Things that the can't quite explain. A hug here, a look there, a desire that wasn't there before. It's not all roses and romance. Sometimes it's pillow fights or blanket forts, arguments over hot chocolate or coffee, early mornings or late nights spent laughing together. This middle part is crucial for the simple reason that this is what makes it believable. If two people hate each other their romance isn't likely to last long. But if they can learn to see from each other's perspective then they might get a little further. Your partner is meant to be your best friend, the person you talk to when you can't confide in others. Even so this part of romance is arguably the hardest to write. It's quite possible that at this point your characters may not even realize they're in love because of how comfortable they've gotten with each other. It's your job as an author to give them that little push to get them to realize it.
If your characters are very in tune with their emotions they may be able to recognize their emotions in the late stages of the beginning. However, the blossoming stage needs to see a strengthening in their friendship. They spend more time together, have more heart to hearts, and learn each other's likes and dislikes.
The Finale
Thane and I had two characters that we thought made the perfect couple. For spoilers sake we'll call them Caleb and Heaven. Now both Heaven and Caleb had very hard life, they understood each other. They could throw insults back and forth and never let the other gain an inch in an argument. There were times they hated each other's guts but they always remained friends. Suddenly we decided to introduce a new character, we'll call him Heath. As we developed all three of their pasts and personalities further we discovered that Heaven and Heath would actually become great friends. Thats when we realized something, While Caleb and Heaven were at each other's side for a long time, they didn't make each other better people. But Heath was able to draw Heaven out of her hatred and anger and encourage her to be a better person.
What I'm trying to say is that the relationships you start out with may not be the one you end up with. And no matter how much you may love the paring (Thane and I still love the AU head cannons we created for them) if they're simply passive, merely existing to be each other's love interests, then they don't belong together. No matter who a person is they will change when around someone they love. They won't be fixed, they won't magically become another person but they will tend to take on different tendencies. Someone who is violent may begin to become more peaceful, someone who is fearful may find comfort or courage. If they refuse to change, then I venture to suggest that they are not truly in love. True, a character who fears love or rejection may settle for a relationship where neither party will change, but their relationship won't seem as real or as fulfilling. They don't need a happy ending to seem happy, as long as they are content in each other's company any situation will be fulfilling for your readers.
It's the little things in love that bring us closer together, and as writers that give us the opportunity to write real, heart wrenching, romance that will inspire your readers to dream.