Why is the tale of Rostam and Sohrab significant to the novel The Kite Runner?

It is in Chapter Four that we are first introduced to the
story of Rostam and Sohrab. We are told that it was Hassan and Amir's favourite story.
It tells of the great warrior Rostam and his fleet-footed horse, Rakhsh. Rostam mortally
wounds his arch-enemy, Sohrab, in battle, only to find out that Sohrab is his long-lost
son. Rostam is of course incredibly upset, especially to hear the dying words of his
son:

If thou
art indeed my father, then hast thou stained thy sword in the life-blood of thy son. And
thou didst it of thine obstinacy. For I sought to turn thee unto love, and I implored of
thee thy name, for I thought to behold in thee the tokens recounted of my mother. But I
appealed unto thy heart in vain, and now is the time gone for
meeting...

This picture of
the grief-stricken Rostam and the dying son Sohrab, who only ever longed for his
father's love, has a massive impact on Hassan, who is reduced to tears practically every
time Amir reads it to him.

The importance of this tale is
of course highly ironic on so many levels. Later on in the novel we discover that Hassan
is actually the secret illegitimate son of Baba. What is interesting is how during the
course of the novel this legend is actually played out again, in a different way and
with different characters, as Amir sacrifices his half-brother to Assef's violence and
does nothing to defend him. Overwhelmed with shame, Amir then completely ignores and
rejects Hassan's friendship and love, even though he, just like Sohrab, only ever wanted
Amir's love and respect. Amir engineers Hassan and his father's dismissal and then has
to live with the guilt of what he has done for a considerable length of
time.

This important legend, then, acts as a kind of symbol
of the relationships and action in the novel, as hidden kinship and relations are a
vital element of what drives the plot forward.