Curse of Hatred

The, 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.

The curse was all but extinguished by the creation of Konoha, with only Madara Uchiha 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, 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, though the attack was set up by Madara's apprentice Obito Uchiha. Despite Itachi Uchiha slaughtering his entire clan to put a swift end to it for good of the Uchiha and the village, Obito, in the guise of Tobi, ensured that Sasuke becomes the sole heir to the curse of hatred.

Trivia

 * According to Obito, Sasuke Uchiha has inherited the hatred of all the Uchiha and as such, is fated to clash against Naruto Uzumaki, who has inherited the Senju's Will of Fire.
 * Although this is said to be the ideal of the members of the Uchiha clan, two known members, namely Itachi and Shisui's ideals were the opposite of this, and seemed to follow more closely to the ideals of the Will of Fire. Obito also followed that ideal before learning of Rin's death and embracing Madara's ideals.