And I said, "Thank you for supper," because that is being polite. (149.45)
This is a perfect example of the way Christopher has learned to interact through rote memorization of social customs. We can just imagine the lesson: "And then when someone cooks you dinner, you should say, 'Thank you for dinner.'" "Why should I say that?" "Because that is being polite."
I wonder if you can understand any of this. I know it will be very difficult for you. But I hope you can understand a little. (157.15)
This line appears in one of Christopher's mother's letters – the one in which she explains why she left him and his father. After going on at length, with heart-wrenching explanations and recollections, she writes that she realizes Christopher might not be able to understand any of the letter at all, suggesting that she's writing this letter as much for herself (as a release valve, as a way to express her own grief and guilt) as she is to apologize to Christopher.