There used to be a thought that in contrast to the Senju clan, who based their strength on love as opposed to jutsu, the basis of the Uchiha clan’s strength was the power of their jutsu.

However, the truth is actually different… There is no clan that feels deeper love than the Uchiha, and that’s why they suppressed and sealed it away.

Once an Uchiha knows love, it’s almost as if all of his or her previously checked emotions are released. They awaken a profound love and power that exceed the Senju’s.

Except that it’s quite problematic. This great power hides within it the possibility of going out of control. When an Uchiha who has known love then loses that deep love, it is replaced by an even stronger hate that changes them.

When an Uchiha individual writhes in agony over the loss of a great love or disappointment in himself, a unique chakra gets released inside his or her brain and reacts with the optic nerves. Changes appear in that person’s eyes.

This is the phenomenon called the sharingan. The eyes that reflect the heart.

Senju Tobirama
NARUTO—Chapter 619

Author’s Note: This quote is taken from the English version of NARUTO VOL. 65 (SHONEN JUMP Manga Edition), translated from Japanese by Mari Morimoto, published by VIZ Media, LCC.

Glossary

Jutsu: technique.

Chakra: The essential essence for the completion of any ninja technique. It’s obtained by mixing two energies, produced by the body (cells) and the mind (spiritual energy). Through various processes, the most common of which is the composition of hand seals, chakra can be manipulated and transformed in such a way as to obtain supernatural effects such as walking on water, spitting fire, or creating illusions.

Sharingan: hereditary visual power of the Uchiha clan giving the eyes of its user a red color and granting them many abilities.