I had one of my characters fall in love with someone else's character, and we actually got to write the end of that RP. Sometimes it turned out really good, other times it was bad. But before I get into that, a word of warning...

Love is cheesy. I got to watch someone close to me fall in love, and now they are happily married. They laughed at unfunny jokes, refused to see each other's faults, and did all that luvy duvy stuff that just turns your stomach. As someone who's fallen in love a small number of times, it doesn't feel cheesy to you. So that's important to keep in mind.

So back to my character, Dedra, and the one she fell in love with, Louis. A few details: She was a baker/militia who had a fear of vampires, he was a vampire that despised his kind. She was serious and stout, he was funny but determined. They both had dark pasts they wanted to escape. They had good chemistry together, and both players wanted to see them together. 

I honestly felt all of the scenes where they just kind of hung out were really cool. Dedra overcome her fear of vampires bit by bit, and Louis... didn't really change, but you discovered why he was the way he was. It was also cool to see how the characters behaved apart. There's a scene I remember fondly that I wasn't even a part of. Louis was offering some soldiers (other players) bread that Dedra made, claiming it was the best he ever had.

...then came the dramatic parts.

Dedra and Louis's first kiss felt forced. they just kinda felt like good friends when it happened. I did like how they were both forced to go on different paths afterwards, and you really got to see how their bonds affected each other. Then Louis turned evil for some random reason, appeared before Dedra as this evil person, and it got kind of out of hand. Thankfully, Louis didn't do anything especially evil, and was able to return to his old, goofy self. That was much easier for Dedra to forgive.

The best part was the climax of the roleplay, which was a battle against Satan. He created these illusions that took the form of the character's past sins. Dedra was chased by all the people she ever let down, and Louis was ambushed by all the woman he had whored around with. With their sins clambering around them, Dedra and Louis had to focus on helping the other one. So as soon as their hands intertwined, the illusions dissipated. That wasn't the only part of the last battle, but it was a nice touch from the GM that showed how far these two characters had come as people. 

TL;DR: It's okay to be a little cheesy, but you need to pick characters with good chemistry and work on those bonds to the point where it feels like something could actually happen. 

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For general emotional writing, it always helps to write about something the reader might be able to relate to. In that sense it's not that different than writing about loss and sorrow. Like in Dedra and Louis's case, their love became more apparent when they were separated or forced into hardships. Despair is a catalyst for many things.