they were each other's firsts to share the same pain and suffering. although Naruto's best friend is Sasuke I always felt that he was closest to Gaara because they understood each other without talking. their actions and decisions makes sense to the other. it still breaks my heart during the kage summit/meeting episodes where Sasuke was aunouce as a wanted ninja dead or alive. naruto suffered greatly trying to back his "friend" up and Gaara said something around "aren't I your freind too" or "freinds don't let each other suffer" (i should go back and watch that episode). naruto will always mean a lot to Gaara. he was his first freind; the first person he opened up too after he scarred his head with "love"; naruto was the person that changed Gaara to be better and because of those things Gaara would come around to help Konoha not as the kazekage, but as a close freind. it was also that point of the series that I honestly started to dislike Naruto a bit.
but I love seeing these 2 together