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Pretext
From the very beginning the focus of this story has always revolved around these two. Their destinies seemed to be to an extent already pre-ordained : sasuke was to follow in the footsteps of madara and naruto was to follow hashiramas path. The curse of hatred and the will of fire, naruto and sasuke, ying and yang.
Obito's explanation for this is that it all stems from the ancient fraternal feud between TES and TYS of the Rikoudo sage.
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With TES's descendants coming to be known as the uchiha clan and TYS's being the senju. Unifying these two bloodlines into one form appears to grant the user two things, the ability to control the gedo mazou and the ability to awaken the rinnegan. However, only one of these abilities seems to come guaranteed:
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the ability to control the gedo mazou.
Awakening the rinnegan seems to require more than senju and uchiha powers. Note that Madara confirms that senju cells are a necessity to awaken the rinnegan, but that having them doesn't necesarilly mean you will. In madara's case, his did not awaken until he was at the verge of death at a seemingly very old age:
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So what exactly is the missing link? What condition did Madara satisfy on the brink of death that allowed him to awaken the rinnegan?
I think we can find the answer to that question by looking elsewhere first, or better yet two "elsewheres" firstSpeculation and Analyses
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The Uzumaki Clan Mask shop in konoha
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The stone tablet of the Uchiha, hidden beneath Naka shrine
When orochimaru went about performing the jutsu to release the reaper death seal and free the hokage, after getting the mask from the uzumaki clan mask shop, he went specifically to the uchiha clans tablet to perform the jutsu. Why would an uzumaki jutsu need to be performed in a location solely associated with the uchiha? The jutsu in question: the reaper death seal, granting the user the ability to channel the god of death to claim a targets soul, in addition to the users own soul.
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This is an ability that can only be really be compared to the human path
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and the preta path's king of hell technique.
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No other sealing techniques targets the human soul so directly or calls forth a being that is a God of sorts in its own way. Simply put, the level of the jutsu is only comparable to rinnegan level jutsu, giving the user the power of a god and the ability to control seal a soul. I believe that is why the mask needed to be in the presence of the uchiha monument to activate. The ties between the shinigami, the sage and the uzumaki are probably revealed on the monument as well.*(Not entirely related to the point of this post but is isn't it peculiar the resembelance between the white flame like orbs surrounding the shinigami and those that the juubi jin has?) *
Speculation & Analyses: Conclusions
I believe the uzumaki clans powers are the catalyst required to activate the rinnegan. It is impossible to say with precision how old Madara was when he awoke the eyes but based off his appearance he was likely far older than hiruzen lived to be, potentially around the age of mito uzumaki ( who lived from aprox the age of the 1st hokage well into the reign of the 3rd hokage ). I think that when Madara transplanted hashiramas cells into himself and the rinnegan did not awaken, he panicked. He would have researched tirelessly to find what he was missing or and done what he could to live long enough to ensure his perfect dream he spoke of would be realized. For this reason, at some point between kushina joining the village and the second great shinobi word war * madara ventured into and destroyed uzushiogakure, looking for the uzumaki equivalent of hashirama cells and at the same time taking the opportunity to eradicate potential future threats. His findings lead him to believe the most capable suitor for his rinnegan would be an uzumaki, trusting an infant nagato with his treasured eyes, putting the entirety of his plans sucess in nagato's unkowing hands. Don't forget, had it not been for kabuto, madara would not have been revived.*there is a brilliant post dedicated to the timeline of these events if you are interested in a more accurate time scale than this
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