A healthy father-son relationship is built on trust, since the older father is in a position of power and authority over the younger son. Without trust, the essential bond can only extend as far as the dishonesty lasts; when the truth comes out, there can be only bad feelings. In The Kite Runner, Baba conceals his parentage of Hassan from Amir both for cultural reasons and because he doesn't want Amir to feel jealous of Hassan. When Amir finds out, many years later, his reaction shows how much of his life the broken trust affects:
How could he have lied to me all those years? To Hassan? He had sat me on his lap when I was little, looked me straight in the eyes, and said, There is only one sin. And that is theft... When you tell a lie, you steal someone’s right to the truth. Hadn’t he said those words to me? And now, fifteen years after I’d buried him, I was learning that Baba had been a thief. And a thief of the worst kind, because the things he’d stolen had been sacred: from me the right to know I had a brother, from Hassan his identity, and from Ali his honor.
(Hosseini, The Kite Runner, Google Books)
Because of this essential lie, Baba's relationship with Amir was always strained; even though Baba's lessons stayed with Amir for his entire life, and even though he found his father admirable in many ways, the revelation of a lie held throughout his entire life taints all of his memories and shows just how much and how deeply Baba hid -- both from Amir, and from himself.