Why do AMir and Hassan react differently to the story of Rostam and Sohrab?

Amir and Hassan share a favorite story: "Rostam and Sohrab" from the Shahnamah. It's a story about a father who kills his nemesis, who actually turns out to be his son. Amir, being the favoured son, sees the story in a different context than Hassan who perhaps defines himself as the son who is killed.

•Amir and Hassan share a favorite story: "Rostam and Sohrab" from the Shahnamah. It's a story about a father who kills his nemesis, who actually turns out to be his son.

•Amir plays another trick on Hassan. Instead of reading from a book, Amir starts to make up his own story, although he flips through the pages as if he's reading from them. Hassan loves it. This prompts Amir to write his first short story.

•Amir, proud of his story, tries to show it to Baba. Baba isn't interested at all; Rahim Khan comes to the rescue and reads Amir's story. Rahim Khan even writes Amir a nice note about the story.

•Emboldened by Rahim Khan's praise, Amir reads his story to Hassan. Hassan is enthralled and even tells Amir he'll be "a great writer" (4.55). However, Hassan points out a plot hole. Amir knows Hassan is right, but he's also angry. He thinks some mean things.