The nature of the relationship between these two contemporaries is explored in Chapter Four, and its importance is of course in the way that the relationship between Ali and Baba parallels precisely the relationship between Amir and Hassan. Note what Amir tells the reader about his father and his father's servant:
Ali and Baba grew up together as childhood playmates--at least until polio crippled Ali's leg--just like Hassan and I grew up a generation later. Baba was always telling us about the mischief he and Ali used to cause, and Ali would shake his head and say, "But, Agha sahib, tell them who was the architect of the mischief and who the poor labourer?" Bab would laugh and throw his arm around Ali.
Although this sounds more like a very close friendship than anything else, Amir then says that never did Baba refer to Ali as his "friend." In addition, Ali is a Hazara, and Baba a Pashtun, so they come from different tribal backgrounds, and the Hazaras were oppressed and discriminated against in Afghanistan. So, although the relationship between Ali and Baba is very close, there are also a number of barriers that prevent it becoming a friendship between two equals; just as in the case of Amir and Hassan, it is always Baba who was in "charge" of the relationship, and Ali who followed after as if he were a servant.