Redemption isn't possible in this case.
-- Narrator (chapter 1)

Importance: At the start of David's first person account, he explains his reasons for being resigned to his fate. Although he insists he did not kill his son Matthew, David feels there is no point in contesting his culpability or his prison sentence. This is because David is weighed by guilt, blaming himself for failing to protect his son. His remorse and shame therefore convince him that he cannot be redeemed for his failures.

Cheryl is trying to move on.
-- Rachel (chapter 2)

Importance: When Rachel shows David the photo of the boy they think is Matthew, one of the first questions that David asks is if she has shown his ex-wife, Matthew's mother Cheryl. Rachel has not done so, as she explains in this line of dialogue, because she is trying to protect her sister. Rachel does not want the photograph to undo or counteract her sister's attempts...