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.

Trivia

 * According to Tobi, 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, a few known members, namely Itachi, Obito and Shisui's ideals were the opposite of this, and seemed to follow more closely to the ideals of the Will of Fire.