Development is the most important part of a believable relationship. You must start at friends, enemies, strangers, etc before you start the relationship. If you have them fall in love too fast, it isn't believable, and if you start with them already in a relationship, for most books there is nowhere to go after that.
Of course, there are exceptions. If something happens to break them up or cause rifts, or the plot is not relationship based, meaning the main focus of the book is not the relationship but something else.
Make the relationship complex, as well. Was it awkward once they got together, if they were friends? How long did they like each other before they got together? Who liked who first? Were they in denial? Did they hide their feelings? If they were enemies, how did they start to like each other?
There are many things to consider when writing a relationship, and the more thought you put into it, the more your audience is going to cheer and feel for your couple.
The main thing: do not rush your relationships. You have all the time in the world to create your characters and let them fall in love naturally.
Some of the relationships listed below are toxic relationships and should not be romanticized. I have a chapter about this also, called "difference between portraying an abusive relationship and romanticizing one."
EXAMPLES
- Ross Gellar and Rachel Green
- Monica Gellar and Chandler Bing
- Lucy and Ricky Ricardo
- Daphne Bridgerton and Simon Bassett
- Cindy and Dean (Blue Valentine)
- Quentin Coldwater and Alice Quinn
- Quentin Coldwater and Eliot Waugh
- Blair Waldorf and Chuck Bass
- Serena van der Woodsen and Dan Humphrey
- Lucas Scott and Peyton Sawyer
- Nathan Scott and Haley James Scott
- Brooke Davis and Julian Baker
- Quinn James and Clay Evans
- Lorelai Gilmore and Luke Danes
- Mitch Pritchett and Cam Tucker
- Jay Gatsby and Daisy Buchanan
- Jack Dawson and Rose DeWitt Bukater
- Noah and Allie (the Notebook)
Yes, that is a lot of One Tree Hill, but I firmly believe that that show is the best out there for showing complex characters and relationships and real life issues.