It's a trait shared by the Indra's. Madara resisted, Sasuke resisted for 4 years?He acknowledged him and treated him as an equal instead of a monster and displayed that by helping the Bijuu.
^ It's interesting how the only being Naruto couldn't TnJ so easily was Sasuke.
Natsu vs Naruto = my dream fanficyeah, guess so. I wonder what would happen if Naruto and Fairy Tail combined... mayhem, that's what I say.
^thisObviously because Naruto was the first person Kurama saw that didn't view the beasts as weapons of mass destruction, but instead living beings. Since he was completely different than each line of Shinobi that came before him in that regard (even Hashirama) Kurama acknowledged him.