Why would BAba have been ashamed of himself regarding Hassan and Amir?

While picnicking, Amir blurts out that Hassan was his brother. Sohrab is surprised: Hassan never said he had a brother. Amir explains. At first, Sohrab doesn't understand why people would "hide it from Father and you" (24.112). Sohrab catches on pretty quickly: Hassan was a Hazara. Neither mention that Baba betrayed Sohrab's grandfather.