In answering the question, I would consider the words of Rahim Khan in the opening of the book. The first chapter features a conversation between Amir and Rahim Khan that both launch the flashbacks and help to fully understand how these words address the question:
Rahim Khan had closed the conversation by saying cryptically, 'There is a way to be good again.' The old man's words cause Amir to remember people and a host of memories - 'Hassan...Baba...Ali...Kabul... the life (he) had lived until the winter of 1975 came along and changed everything.'
The desire to "be good again" takes on a larger significance. While one might escape their transgressions and issues that are unresolved, these elements remain unsettled. In order to resolve them to a point where individual consciousness must confront these issues and struggles. Amir might have settled in America, found a life away from Afghanistan and Kabul in the mid 1970s, but it had never been forgotten. The resurgence of the memories that awoke with Rahim Khan's phone call helps to demonstrate this. Amir could not put these aside and recognized the need to confront these memories of the past in order to "be good again."