1. Because Naruto has a big crush on her that developed into love after he started understanding her.
2. With Naruto, Sakura doesn't need to be all nervous and put up a front. None of that sweet perfect girl stuff.
3. They seem to understand eachother. Through the toughest of times that they go through, they'll always be there for eachother.
4. How much Naruto takes care of Sakura. He was there for her in the Gaara fight and in the episode with that 'amazing' race. He thought Sakura had drowned, and he was feeling panicky, so he ran as fast as he could to get to land. Also in the episode where they go to Orochimaru's place looking for Sosuke, saying to her that "You must not die, Sakura-chan!" and that without her to inspire him he wouldn't have broken out of that pyramid thing.
5. When it actually seems like Naruto is dead or in grave danger, Sakura really worries about him. Infact, she flung herself and cried on him when she thought "Kabuto" had killed him when he put that chakra thread in his heart.
6. Despite all the emotion, NaruSaku ain't all serious. One minute, Sakura will be crying over him, he'll wake up from near death, and she'll hit him in typical anime style.

Gotta love it. It's something which only NaruSaku has.