These are just some thoughts about Naruto and the nine tail fox throughout the series so far.
Sage mode as stated by Pa frog only can be achieved by people who have large amounts of chakra and the gutsiest. Now as we all know Naruto managed to master sage mode, but why was Naruto able to master sage mode and not Jiraya? So the only think I could think of that separated them was the nine tail fox. Naruto actually if you think about it has been training for sage mode his whole life. Naruto’s chakra circulatory system has been trying to find an equilibrium with nine tails fox’s chakra ever since it was sealed in him. I believe his experience in handling the nine tail’s chakra essentially an external chakra source like natural energy, allowed him to handle natural energy easier and hence mastered sage mode. Though the nine tails stopped Pa frog from fusing with Naruto for unlimited sage chakra, but he also helped Naruto in my opinion to master sage mode over the years. You can’t have everything I guess. I reakon this is a far better reason as to why Naruto mastered sage mode other than the fact he’s the hero of the series. And yes I know plot no jutsu is very powerful, but my theory is plausible.
Another instance where I believe the nine tail fox is making life difficult for Naruto (not in sage mode) is the rasengan. Even though he’s capable of making a technique like fuuton rasengan he still needs a clone to form a rasengan. Basically what the fox is doing is fooling around with Naruto’s chakra simply by being present in Naruto’s body. This also would explain why Naruto uses shadow clones a lot to bypass problems when he is forced to deal with high degrees of chakra control. But on the flip side with the ability to make so many clones he can train a lot faster than anyone else and his skill in using shadow clones increased through repeated use which allowed him to add wind chakra to the rasengan, again you can’t have everything.
Here are pages from the manga regarding the fox making it difficult for Naruto to control his chakra.
Finally I’d like to sum up what I think is happening between Naruto and nine tails there are the obvious advantages the nine tails gives, stamina, regen, strength and speed etc. But also he tries to make Naruto dependent on him by being a burden and not allowing Naruto to use his chakra effectively. And the reason he wants to that is simply to be released, based on how the seal works it allows the nine tails’ chakra leak from the seal and mix with Naruto’s over time. I also believe that “jutsu” the Forth left for Naruto to complete will accelerate this process. So my theory is the fox realizes he’s fighting a losing battle for control and eventually all of his chakra will mix with Naruto’s, which would explain the Nine-Tail’s reluctance to aid Naruto in battle and desperately trying to trick Naruto to take the seal off his cage like when he was fighting Pain.
Here are pages referring to the nine tail's chakra leaking from the seal and mixing with Naruto’s.
Here is a page regarding the nine tail fox's reluctance to aid Naruto.
And finally here is a page outlining the nine tail fox's desperation and anger when the forth intervened just in time to stop Naruto from releasing him.
That’s pretty much all I wanted to discuss, Lets us know what you guys think.