Love Interests are Allergic to Sequels

This cliche can both provide a strong emotional incentive for the hero, and can divorce the hero from any attachments, leaving them free to go out and kick ass with abandon. Because plots are driven by conflict, it is not common to see a happily united couple stay together for any extended period of time. Even if they've hooked up at the end of the movie, the thrill of the chase is over and the easiest way for writers to shake things up for a sequel is to dispose of one of the unfortunate lovers. Especially prevalent in action movies, where having a love interest would slow down the action.

Can also occur in TV shows, where the main characters constantly have new romances from episode to episode.