Sasuke's early childhood, as told over the course of Naruto manga volume twenty-five, was spent living in the shadow of his older brother, Itachi Uchiha, a natural prodigy of the Uchiha clan whom Sasuke looked up to and admired. In an effort to win some recognition for himself, Sasuke trained diligently to win some of his parents' attention, though he was never able to surpass the milestones set in place by his brother, who would be the only one to acknowledge Sasuke's accomplishments. In time, Itachi began to distance himself from the clan and fell out of his family's confidence, which consequently began to look to Sasuke as the new future for the clan.
Soon after Sasuke started to be accepted into his family, he returned home one day to find the streets littered with the corpses of the Uchiha clan. Upon rushing home, Sasuke found Itachi standing over his parents's corpses. As Sasuke tried to flee, Itachi went on to explain that he had never loved his little brother. Claiming that Sasuke was not worth killing, Itachi encouraged him to get stronger by living a life of hate and anger towards him, with his sole purpose being to avenge the clan. Itachi left, and in the absence of a loving family Sasuke agreed to do as instructed, dedicating his life to killing Itachi. Years later, in Part II of the series, Sasuke is shown to have some doubts about the events of that night. It is not until after Itachi's death that he learns the truth surrounding the massacre of the Uchiha: that it was all done on command by the village, and that Itachi had left him alive because he could not bring himself to kill him.
When first introduced during his assignment to Team 7, Sasuke displays a great indifference to his teammates. Feeling that his abilities are far superior to their own, he is unwilling to cooperate with Naruto Uzumaki or Sakura Haruno, as doing so would do nothing to help him kill Itachi. These perceptions are quickly proven wrong, for despite their failings, Sakura is an excellent source of information; while competition with Naruto acts as an effective way to get stronger. While he retains a confidence in himself throughout the series, Sasuke increasingly relies on his friends over the course of Part I. As he starts to get close to others he also begins to risk his own life to save them, even though his death would leave him unable to fulfill his goal of vengeance upon Itachi.
Although Sasuke becomes content with a life of happiness in Konohagakure, he never allows his ambition to gain power to leave his thoughts. During his Part I battles with characters such as Haku and Gaara, Sasuke tests his abilities against those of progressively stronger ninja, in the process discovering the weakpoints he needs to overcome. While initially complacent with his development, Naruto quickly starts to grow stronger at a faster rate. This, coupled with his quick defeat by Itachi during a brief return to Konoha, leads Sasuke to believe that his growth is slowing. In an attempt to reassess his strength, he begins to treat his friends as opponents so as to test his abilities against their own.
Dissatisfied with his progress in Konoha and believing Orochimaru holds the keys to getting strong enough to kill Itachi, Sasuke defects from Konoha at the end of Part I. Naruto attempts to stop him, but Sasuke, believing the death of Naruto, his closest friend, will give him the power he desires (as he was told by Itachi), tries to kill him instead. He is ultimately unable to follow through with this idea, because he felt that is what Itachi would have wanted him to do, and instead continues on to Orochimaru. In the two-and-a-half years he spends with Orochimaru, Sasuke's goal to kill Itachi takes precedence above all else; he even claims he is willing to give his body to Orochimaru if it means killing Itachi, but later turns on Orochimaru after reaching the conclusion that he can learn nothing else of value. After achieving his goal of bringing about Itachi's death, in the process learning how his brother was manipulated by Konoha's higher ups, Sasuke abandons all of his hate for Itachi and resolves to kill Konoha's higher ups for both of their sakes. he has been consumed by hate and lives a life of revenge.
from : wikipedia.org
