There are a couple of themes that the story "Sohrab and Rustum" addresses. First, the legend certainly discusses the irony of life. For much of his adult life Rustum seeks his long-lost son Sohrab, and just as he finds him, he discovers that he has fatally wounded Sohrab. While this type of situational irony plays a key role in many legends and other forms of fiction, reality also offers its own coincidences.
Secondly, the tale portrays the effects of pride and anger. If Rustum had not been so concerned with his reputation as a warrior and so controlled by his pride and rage, he might not have engaged in such a bloody fight with the other warrior who turns out to be Sohrab. But, as many fables and tales teach, pride often leads to destruction, and in this case, it results in the unhappy ending to Rustum's one quest in life--finding his son.