Board Thread:Theories and Speculation/@comment-890307-20161205161301/@comment-734582-20161218162806

Lorenzo VonMT wrote: Remember when madara took kakashi's mangekyo? When he transplanted it into his empty eye socket, the pattern of the sharingan didn't merge with madara's pattern, it stayed the same. Which means that it didnt become an EMS. True, but there's also no mention that it's an instant awakening. He possesses the eye in his socket for about 10 seconds or so. Sasuke on the other hand was out of it for at least a few hours before showing his EMS, and possibly even more.

And as Ravenlot mentioned, that assumes that Obito would've been compatible with Madara for making it an EMS.

Ravenlot 27 wrote: Well, what really matters is the fact the eyes are cured from the blindness, not how they are cured. True. Still if we consider what we know of transplanted eyes, namely that the original owner's chakra still resides in it (E.g. with Danzo), i think the reason the light fades is simply that the Uchiha's own chakra isn't strong enough to maintain the power. A close enough sibling (and considering what we know of Nagato/Kakashi, a long enough transplant) can synergize with the user's chakra.

The changed pattern simply shows that the original's chakra is now merged with the new owner's chakra. Somehow this combined chakra is strong enough for the light to never fade.

Alternatively, there's some minor personal flaw in each MS and only siblings are compatible enough to awaken EMS yet different enough to not share the flaw, ensuring eternal light.