Curse of Hatred

The, is an ideal that is said to have been passed down from Indra, the Uchiha clan's forefather, to his many descendants. The origins of this way of thinking can be traced back to the time of the Sage of the Six Paths, when on his deathbed the forefather of ninja chose to name his younger son his successor over his older son, despite the elder's supposed birthright. The Sage chose the child he thought most suitable to continue his legacy. Asura, who believed love was necessary to achieve peace, over Indra, who sought power to bring peace. Consumed by hatred and envy after their father's death, Indra — manipulated by Black Zetsu — waged war against his brother, and began a cycle of conflict that transcended time and culminated in mutual hostility between the brother's descendants, the Uchiha and the Senju.

As was revealed by the Second Hokage, Tobirama Senju, the Uchiha are people greatly devoted to love and friendship. Because the Uchiha tend to conceal their more affectionate traits, a member's love can be transformed into hatred in the blink of an eye if they somehow lose those closest to them. The despair of the experience causes a manifestation of unique chakra within that Uchiha's brain, impacting the optic nerves and creating the Sharingan. Furthermore, advanced forms of the Sharingan require extremely painful experiences that cause the user to delve into darkness that utterly consumes them. This causes the Uchiha to do anything in their power achieve their goals and showing their own superiority. An Uchiha who falls victim to the curse can give up that hatred if someone close to them can reach them however, though this is incredibly difficult as the Uchiha becomes largely unreasonable and unflinching in their singular desire. For example, Madara Uchiha succumbed to the curse after cutting off his friendship with Hashirama Senju, then gave up that hatred when he acknowledged Hashirama's willingness to kill himself to bring peace between the clans, only to fall once again when learns that the villagers and his own clan didn't trust his warnings about Tobirama.

Over the years, there have been several Uchiha who managed to transcend the limitations of acting only for the sake of the clan, and instead valuing Konoha as a whole. Uchiha including Kagami — a subordinate of the Second Hokage, his descendant Shisui, and Shisui's best friend, Itachi, all inherited the Will of Fire, acting in the best interest of their village as opposed to themselves.

Trivia

 * It has been noted that the Senju clan's Will of Fire is fated to clash against the Curse of Hatred and Obito believes that Sasuke Uchiha and Naruto Uzumaki, both respectively embodying the polar ideals, are destined to fight to the death.
 * Kakashi theorised that Obito never lost his Will of Fire despite falling to hatred, and the two conflicting ideals is what ultimately prevented him from completing the Eye of the Moon Plan when he had the chance.