Naruto's return.

"Sakura and Kakashi? Those two have been living in their own world since Naruto and Sasuke left." Tsunade casually said. "They're even betting on which one of them gets to ground Sasuke for being immature and arguing over whether or not they'll adopt Naruto."

Because Kakashi and Sakura weren't like other couples. They contradict everything, even destiny, much to people's shock.