He's angry...at Wenny for leaving, his parents for being so lost in their grief, and himself for letting Wenny accompany him to the store that day and for not being able to save her. His parents are incapable of helping him, so it is a youth minister who finally suggests that Will start writing letters to God. Instead, he chooses to write to Wenny.
This story is a tear-jerker, but it's also a message of hope and redemption. I recommend that librarians and counselors read it, and add it to their bibliotherapy toolbox.