The Curse of Hatred (憎しみの呪い, Nikushimi no Noroi), is an ideal of enmity that was said to have been passed down throughout the generations by the Uchiha clan's forefather to the members of his lineage. The origins of this curse can be traced back to when a dying Sage of the Six Paths took the fateful decision to name his younger son, who would go on to become the forefather of the Senju clan, his successor instead of the older sibling because the Sage believed that the beliefs held by the younger son were the key to peace. Consumed with hatred, the elder brother eventually waged war against his brother and thus began a cycle of hatred that transcended history, culminating in the mutual hostility between the Uchiha and Senju.
As Tobirama Senju revealed, the Uchiha treasured love and friendship even more than their own clan, but, in the end, that was their downfall. Because this was a part of them they regularly sealed away, when an Uchiha would happen to lose their loved ones, that love turns into a stronger hatred that completely changes the person. By experiencing that loss or despair, special chakra sprouts in the Uchiha's brain, affecting the optic nerve and producing a change in the eyes: the "eye that reflects feelings", the Sharingan. That power gets stronger through the Uchiha's feelings, and drives them into an even deeper darkness, making them impossible to handle, as in Madara's and Sasuke's case.
The curse was almost extinguished by the creation of Konoha, with only Madara refusing to relinquish it. Infuriated that his rival Hashirama Senju was made First Hokage instead of himself, Madara sowed the seeds of contempt among his kin with the promise that they would eventually lose standing in their partnership with the Senju if they did not help him. Though Madara was exiled by his clan soon after, a few Uchiha would begin to believe his warning generations later yet were not in a position of power to do anything about it. The Uchiha's hatred was fully rekindled later as the result of them being unjustly accused for the Nine-Tails' Attack on Konoha, which in reality had been set up by Obito Uchiha. Years later, Sasuke Uchiha would come to inherit this immense hatred as well for what was done to his clan and brother, the involvement of Konohagakure in his clan's demise being revealed to him by Obito. Obito noted that Sasuke has inherited the hatred of all the Uchiha and as such, was fated to clash against Naruto Uzumaki, who has inherited the Senju's Will of Fire.