First introduced in the manga, Uchiha seems like a quiet, somewhat weird, and stuck-up kid, with the best grades, crushes of all the girls in his class and envied by all the boys. It's only when you have watched him work together with his team for a while can you understand the motives behind his actions, and a completely new character is revealed.
To understand Sasuke's character, one must understand that he survived a tragic past; at the age of 8, his one idol betrayed him and murdered his clan, his relatives, his parents, right before his eyes. These circumstances pushed his young soul towards one goal: to avenge his clan, and kill the one person who is responsible for his loneliness.
Yes, he's a really lonely kid. He acts the way he does because he made himself believe that revenge was the only way for him to live his life. After his parents' deaths, no adult takes care of him, he lives on his own, and nobody tells him what is right and what is wrong. He doesn't want to lose any more loved ones, so he avoids making friends, and he concentrates on becoming a better ninja; he is isolated from the rest of the society.
Everything changes when he is put in a 3 man team with a teacher. Sasuke, while not even realizing that he's doing so, motivates his two classmates, Naruto and Sakura, to become better ninjas. Naruto, who always tries to prove that he's better, declares Sasuke his rival and pushes himself further than ever before, to become stronger. Sakura, to prove to Sasuke that she is worthy of his love, does the same.
Sasuke comes to care deeply for his teammates - this is demonstrated again and again when he puts his life on the line to save his teammates
Spoiler Alert! Click to show or hide
(he jumps before Naruto so he wouldn't get hit by Haku's needles, he tells Naruto to save Sakura while he stays and fights with a jinchuuriki, and he goes after Naruto blindly when he hears that the man who murdered his parents is after him)
However, a change in his character starts the moment he is defeated by the person he tries to kill - he realizes that Naruto has become stronger than him, and the feeling of helplessness, the feeling that he has been wasting his time drives all rational though out of his mind. He forgets caring for his teammates, banishes all feelings beside hatred, and leaves his friends.
In three years time, when he appears again, it seems like Sasuke has completely and irreversibly changed into a cold-blooded murderer; not even killing his best friend is a sin anymore if it gets him closer to his goal of killing his prey. Only after he succeeds in his revenge, and the tragic past is revealed to him, does Sasuke understand the sacrifices made so his life would be spared, and his soul is thrust into even deeper darkness.
Since his motives at this point are unclear, I am forced to finish this review. I would like to ask anyone reading, please refrain from calling Sasuke an "emo kid". That is incorrect, and by calling him that, you only prove how much you don't pay attention to what you are reading/watching, and you insult the man who created this character and the whole show.
Thank you for reading.