Who says "I opened my mouth and almost said something... The rest of my life might have been differently if I had ." Why are those words important? What do you think of Amir's behavior?

Amir is taking about his cowardly not to help Hassan when Assef molests him. Amir will carry this guilt for much of the book. It will eat at him from the inside out. I think Amir was a boy at the time and Assef terrified him. I don't judge Amir for not helping his friend but I can certainly understand his guilt.