Why does Amir avoid answering Farids questions about why he has come back to Kabul for this particular boy?

Amir avoids answering Farid's questions about his return to Kabul for the boy because he feels a deep sense of guilt and shame about his past actions, particularly regarding Hassan. Discussing the reasons would force Amir to confront his painful memories and the betrayal he committed, which he is not yet ready to do. Additionally, he may sense that revealing too much could expose vulnerabilities that he wishes to keep hidden in a war-torn environment.