How is Hassan's reaction to the story a metaphor for Amir's life?It's talking about Amir's first story.
While Hassan does indeed praise Amir for his story, he also points out a significant plot hole in it. He asks why the main character didn't just smell onions to get himself to cry (instead of killing his wife). Amir recognizes the logic behind Hassan's question, but he doesn't appreciate being questioned by an "inferior" Hazara.
Hassan's positive reaction to Amir's story represents his infatigable loyalty to Amir, even after Amir's betrayal of him in Chapter 7.
Hassan's questioning Amir serves as a theme of Amir's being adverse to criticism, but it also represents that while Hassan seems to always look for the most logical and least harmful solution to problems, Amir is willing to take the quickest route to reach his goals--even if it means hurting those around him.
Hassan's reaction to hearing the reading of Amir's first short story is that of extreme exhuberance. He predicts that Amir will become a famous writer some day, and Amir's life eventually unfolds just in the manner that the simple Hassan predicted. Amir did become a successful writer, but writing was not Amir's problem; he was always able to tell a tale and write. It was the aspects of his life that he could not control that plagued him: His indiscretions against Hassan, his relationship with his father, and later, the secrets that Baba kept from him were elements that continued to haunt him.