It's been foreshadowed at the beginning of the series and it's been foreshadowed in the war when Minato compared Sakura with Kushina and all in between. Kushina's last words to Naruto before she died were "well since I'm a girl I don't know much just find one just like your mother!" Hitting in anime shows love not just a comedy aspect. Yamato in the 2nd archive of the series pointed out that Sakura really cared about Naruto. In the movie Bonds Sai said to Sakura "come to think of it in a book that I read it says; women become infinitely strong for the man they love." After Sakura said she was going into battle. To which Sakura yelled at him.