Here's some reason:
- reunited each other after the battle, etc.
- confess their feelings after loving each other
- either months, years or few weeks/days, they decide to get married after loving each other and decide to be together forever
- sometimes they put on hold after they know each other more
- sometimes we have to wait until the sequel so we know that they're getting married or not yet
- No matter where you're from, what you look like or redeemed yourself....love is more important thing
- Princess Fiona and Shrek are married in the first movie after confessing their love.
- Jason Johnston and Evelyn Ross got married three years in the future, according to Seth Wilson's vision.
- Robin Hood and Maid Marian got married after Prince John, Sir Hiss and the Sheriff of Nottingham are thrown in jail.
- Big Gay Al and Mr. Slave got married at the end of South Park episode "Follow That Egg!".
- Santana Lopez and Brittany Pierce got married in Glee episode "A Wedding".
- Kurt Hummel and Blaine Anderson got married in final season of Glee.
- Muscle Man and Stara got married in Regular Show episode "Dumped at the Altar".
- The Great Leslie and Maggie DuBois are married after the "New York to Paris" race ends in draw with Professor Fate demanding a rematch a race back home to New York.
- The Undertaker and Michelle McCool are married in 2010 after their meetings in 2007.