When Naruto loses control of the nine-tailed fox's chakra and kills Gaara in the battle after the third chunin exams the moon decides to make Gaara a new guardian.
Pitch Black(boogie man) has returned to take revenge and has gone to the Naruto universe to bring back extra help to defeat the guardians but he ends up bring the sixth guardian with him and it's up to the guardians to stop him. (sorry I suck at summaries) I don't own Naruto or Rise of The Guardians
Could it have been a mere coincidence that Jack Frost and Naruto Uzumaki came together. Everything they been through caused by a stroke of luck. They only just met and they already call each other family. Through the moons light this was meant to be.