because there is someone.. I want to protect

After Naruto had won the first as well as the second round against Kiba and Neji (which was simply stunning and also moving, seeing Naruto getting all respect of all the people who watched the fights!), Gaara and Sasuke's fight took place. However, as we know, the fight between Gaara and Sasuke got .. let's say "interrupted" by some unforeseeable events. Sasuke though, chased after Gaara to finish their battle, which was in fact not a too good idea. The cursed seal had spread across Sasuke's body and therefore worsened his condition drastically. Sasuke was at his limits and Gaara about to kill him - but right in time Naruto came to save the day :D

Once Naruto saw Gaara, he was shocked, and one can't deny he wasn't scared at that moment. Gaara started to turn more and more into the monster sealed within him, and just as he wanted to scream they should run better instead of fighting, Gaara tried to kill Sasuke for the 2nd time.
Naruto knew he wouldn't make it in time, but in the last second Sakura defended Sasuke, which spared his life but got her captured by Gaara's sand and knocked unconscious. Naruto didn't have the slightest clue what do to, seeing as he wouldn't stand a minute against Gaara, but he simply had to do something in order to save both of his teammates.

So the situation seemed anything but good for Naruto, until Gaara stated he wouldn't become any stronger if he continued to care for "strangers". Naruto then remembered back how it was when he still was alone, just like Gaara still is. He overcame his fears and decided to take Gaara on, to make sure to prove his theory of getting stronger is wrong.

Naruto once again amazed Sasuke by his sudden change of energy- he fought like he had never done so before, just to protect his friends. Sasuke then told him he should get Sakura and run as fast as possible, while he'd try make up some time. Sasuke then asserted he'd never ever want to lose someone precious again, professing that Naruto was indeed his very important friend.
Naruto was honestly surprised to hear those words from Sasuke, since it was the first time Sasuke told him something like that; he considered him as a real friend. Thus he was now ready to defeat Gaara no matter what in order to protect the people dear and precious to him.
Sasuke's words must have awaken something deep down in Naruto's heart, since he suddenly was full of energy again and tried his best to win the battle. Sasuke couldn't believe his own eyes while watching Naruto, who suddenly seemed to be so much stronger than before.

I'm pretty sure these episodes/chapters got rid of many prejudices concerning Sasuke, as well as Naruto's and Sasuke's relationship. Sasuke admitted for the first time that Naruto, as well as Sakura, are very important to him, and that he doesn't want to lose someone important to him again. That's why he's also ready to put his life on the line. Naruto was clearly struck by this words, which gave him new energy and spirit to protect his teammates and, of course to show Gaara his way of getting stronger is wrong.

Compared to the fight in the Haku arc, this time, Sasuke's mind could name what exactly made him try his fullest to protect Naruto and also Sakura. At that time, in the country of the Waves, he mostly blamed this body for reacting on its own accord; but this time he must have realised how much he trasured both his teammates. Ever since the death of his parents, Sasuke admitted to himself, as well as to Naruto, that he had found someone again whom he was willing to protect no matter what.

Naruto had learned an incredible amount from those words. They simply had a huge emotional effect on him, since it was the 2nd time someone wanted to give his everything in order to protect him (The first time it had been Iruka, in the very first chapter). While Naruto unconsciously taught Sasuke to "love" and to trust again, without doubt also Sasuke taught Naruto the meaning of true strength - he taught him how to get stronger, and thus, without doubt I'd say Sasuke has been the trigger for Naruto's first true shinobi development.

"When a person has something important to protect.. that's when they can truly become strong." Haku, chapter 21

Found at Chapter 14, Episode 76-78