"You have that kind of resolve. Good."
Itachi had awakened his eyes not long ago. After awakening the Two-Tomoe Sharingan as a child, as his chakra and strength increased through constant training,
his Sharingan had gradually evolved as well.
It had soon reached the level of the Three-Tomoe Sharingan, which fully proved one thing.
Awakening an ordinary Sharingan required only a single powerful stimulus.
That was the stage of going from nothing to something.
As for advancing from one tomoe to three tomoe, steady chakra growth and continually strengthening Yin Release were enough, though talent naturally played a part as well.
Some people could achieve it very quickly, while others could never achieve it in their entire lives.
After all, individual talent set the limits. Otherwise, the Uchiha would have long since all become jōnin.
As for Itachi's Mangekyō Sharingan,
it had awakened with Lu You's special help. In theory, Uchiha clansmen could not awaken the Mangekyō without extreme suffering and extraordinary talent.
But Lu You knew.
It was all bullshit!
Even the Rinnegan supposedly required a mixture of Indra's and Asura's chakra to manifest, yet the Sage of Six Paths had handed it out just like that, forcibly upgrading Sasuke's Eternal Mangekyō Sharingan to a Tomoe Rinnegan.
In other words, ocular techniques were the ultimate manifestation of Yin Release. At the end of Yin Release lay the other pole of Six Paths Power-the Rinnegan.
Suffering caused Uchiha clansmen to produce a special kind of chakra. With that chakra's help, the Uchiha could awaken their Sharingan, or even awaken the Mangekyō Sharingan.
That special chakra lay hidden within the Uchiha bloodline.
So, as long as he figured out the true nature of that special chakra, helping Itachi awaken his eyes was trivial.
After all, before the current Lu You, even