A lot of people don't really view the situation in the perspective of Naruto.

Early Sasuke was really the best, and only gets a bad rep because people apply his later actions to his entire character. He looks out for Naruto in the Land of Waves arc, nearly dies for him, acknowledges Naruto as a rival, he cares for Sakura (contrary to what people claim), he is willing to die for Naruto/Sakura against Gaara. Sasuke's jealousy of Naruto first began after the Gaara fight - but he repressed it and it is implied that he did try to seek advice from Kakashi for his stagnant progress.

Sasuke's hostility toward Naruto starts after the Itachi thing; but, Naruto only saw it as a part of their rivalry. He doesn't make a connection between Sasuke's behavior and the Itachi stuff. It isn't until VOTE1 when Naruto realizes Sasuke changed, but he doesn't know why.

Naruto didn't even know about Sasuke's cursemark until the end of Part I.