Hassan is one of the major characters in The Kite Runner, by Khaled Hosseini, and he is certainly also one of the most sympathetic. It is difficult for readers not to like Hassan because he is a kind, loyal, and optimistic friend to Amir; it is almost impossible for readers not to sympathize with him because Amir takes such advantage of those very qualities. Hassan's personality, temperament, and character are things he (for the most part) chooses; however, his childhood and upbringing are beyond his control and in many ways determine his future.
Hassan was raised by his wise, gentle, and loving father, Ali, because Hassan's mother saw her newborn son's harelip, was disgusted, and left. Of course we find out later that Hassan is actually Baba's son, but he is raised and grows up as the son of a servant. Though he is illiterate, Hassan is quite intuitive and perceptive; unfortunately, his natural intellect does not get him as far as an education would have. Hassan is poor but does not spend his time or energy fretting about that; instead he spends his childhood serving (and being ill-treated) by his half-brother Amir.
In the Afghanistan culture, Hassan has very little chance of ever being anything much more respectable than a servant. (We see plenty of examples in the novel of people falling from high places but virtually none who elevate themselves beyond the station in life to which they are born.) Being born the son of a servant (despite his biological father's wealth and standing) limits his choices of careers and wives.
What affects Hassan even more is the events of his childhood. If he had not devoted himself so loyally to Amir, if he had not been willing to be Amir's kite runner, if had ever put himself above Amir, if he had stood up for himself when Amir boldly told lies about him, things might have been different for Hassan. Instead, his unflinching loyalty is what kills him, as he comes back to Kabul to help Rahim Khan care for Baba's house. If he had stayed in his remote village, it is not likely he would have been killed. The habit of loyalty, begun in his childhood, was the cause of his death.