The Curse of Hatred (憎しみの呪い, Nikushimi no Noroi; ; ; ), is an ideal that is said to have been passed down from the Uchiha clan's forefather to his many descendents. 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 forbear 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 (the younger, who believed love necessary to achieve peace, over the elder, who sought power to bring peace). Consumed by hatred and envy after their father's death, the elder son waged war against his brother, and began a cycle of conflict that transcended time and culminated in mutual hostility between the brother's descendents, the Uchiha and the Senju.
As was revealed by the Second Hokage Tobirama Senju, the Uchiha are a people greatly devoted to love and friendship; so much so that it most often becomes their eventual downfall. 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, which Tobirama literally referred to as "the eye that reflects feelings". Furthermore, the techniques known to cause more advanced development of the Sharingan require extremely painful experiences that cause the user to delve into darkness that utterly consumes them. Prior to the founding of the Konoha, Madara Uchiha was one such shinobi. Madara refused to relinquish his hatred for the Senju after being forced to take and transplant his beloved brother's eyes in order to obtain the Eternal Mangekyō Sharingan. Unsure how long the Uchiha's standing in the Konoha would last after the Second Hokage replaced the First, Madara sowed seeds of contempt amongst his brethren, swearing that they would surely lose their partnership with the Senju if they did not help him.
As an Uchiha delves into the darkness of the Curse of Hatred, even the slightest hint or mention of anything even remotely connected to the person whose death caused them to lose themselves will make them extremely angry and emotional, as was seen in the cases of Madara, Obito and Sasuke Uchiha. Also, after having delved, an Uchiha cannot be reasoned with by anyone except the person who originally caused them to do so. Only that person has any chance of saving the Uchiha from the curse, as was seen in Sasuke's case.
Though Madara was exiled by his clan for attempting to disrupt the peace they had all worked hard to achieve with the Senju, generations later, a few members finally took heed of his warning. It was too late, however, as the Uchiha were no longer in a position of enough power due to the Second Hokage's attempts to thwart the Curse by designating the Uchiha to the Konoha Military Police Force in order to regulate their emotions. Later, the Curse was fully rekindled after Obito Uchiha witnessed the death of Rin Nohara. This event caused him to orchestrate Nine-Tails' Attack on Konoha, which resulted in the Uchiha plotting to take over Konoha after being unjustly accused of setting the demon on the village. This led to the massacre of the Uchiha clan at the hands of Itachi Uchiha, which years later led to Sasuke Uchiha's quest for avenging his family resorting in becoming what Obito noted as the sole inheritor of the Uchiha clan's hatred and thus fated to clash against Naruto Uzumaki, who has inherited the Senju's Will of Fire. However, questioning his resolve, Sasuke has Orochimaru reincarnate the Hokage and learns the full nature of the Curse of Hatred from Tobirama. Having also learned from the past Hokage about the origins of Konohagakure, Sasuke choose to protect the village, and his brother's sacrifice instead of destroying it.
However, throughout generations there were Uchiha who were able to transcend the limitations of thinking only of their clan and valuing the village as a whole. Uchiha such as Kagami — a subordinate of the Second Hokage, his son Shisui, and Shisui's best friend, Itachi all inherited the village's will to care for and look out for its well-being.