[THEORY] The Juubi's Chakra Is The Source Of Sharingan
I'm just going to jump right into it.
I believe that when the Juubi was sealed inside of RS, RS obtained all of its chakra.
When RS passed his inheritance down to his sons, the body and the eyes, each obtained a different type of chakra.
The brother who got the eyes obtained the Juubi's evil portion of chakra; while he might have been able to embrace, the later generations (Uchiha) would repress this chakra until it is unlocked later by some traumatic event.
Now, I've heard people discount this claim because the Juubi didn't feel "hate". In fact, it had no feelings or emotions.
But wouldn't this just play to the Juubi-Uchiha theory anyways?
The Juubi was a completely irrational beast.
So, is it just coincidence that every time an Uchiha obtains a new level of Sharingan, they become less and less rational?
Let's take a look at the recent Madara. Everything was going fine and dandy until someone tried to attack his beloved brother. He awakened his Sharingan, and then he made the IRRATIONAL descision of throwing away everything he and Hashirama had worked for.
Similar themes can be seen in Sasuke and Obito and their progression of the Sharingan. Obito initially wasn't irrational when he awakened his 2 tomoe; but with a nudge from Madara- Well, you know.
Same with Sasuke. As Sasuke progressed further with his Sharingan, he made more and more irrational descisions. After 2-tomoe, Sasuke grew incredibly jealous of Naruto and even went for a kill shot during their duel. At 3 tomoe, Sasuke adopted the descision to forever break bonds with the Leaf, not to just leave. The most irrational descision of all was to join Orochimaru, in fact. Moving on to MS, Sasuke went batsh*t frickin' crazy. The only reason EMS didn't completely take him over was because of the intervention of Itachi.
Which brings me to my next point! Itachi. One could argue Itachi didn't succumb to any of this, and still developed his Sharingan. Which would be a valid point. My only problem with this is that Itachi was just an EXCEPTION to the rule; he acknowledged the fact of what the Uchiha were, and overcame this. (I think a similar case will happen to Sasuke in the following chapters).
But what's the point of all this? The Juubi. I think the Uchiha's Sharingan has roots in the Juubi's chakra itself, which is why they become more irrational as they progress through the Sharingan and unlock more of the Juubi's chakra.
Now, that was longer than I intended, so...
The Sage split the Juubi's two abilities to his sons, it's chakra (eyes/sharingan) and it's natural energy (body/mokuton). The Uchiha's Sharingan comes straight from the Juubi's chakra; as they unlock later versions of the Sharingan, they obtain more of the Juubi's chakra. This is also why as the Sharingan continues to grow, the Uchiha's descsions become more and more irrational.
Don't do many theories, so any tips would help and how to construct them for future?...