Wolff is of two minds about the ending. On the one hand, we wants us to know that his life still has plenty of troubles… specifically by telling us that he got kicked out of prep school when "I broke the bank and was asked to leave" (31.11) and, even scarier, joined the Army just in time for Vietnam. There's also the fact that his father gets tossed in the nut hatch, which Jack finds out about "when his girlfriend called to say that he had gone crazy." (31.3) And the guy who's supposed to be taking care of him at his father's "embracing me and making declarations of love." (31.1) Even though Dwight's gone, Jack's life isn't getting any easier, and the dysfunctional wackiness of his early years will continue after we've read the final page.
But then after telling us all that, Wolff backs up to talk about the drive back to Chinook with Chuck before he leaves for prep school. They're happy. They're singing songs on the radio. Chuck isn't going to jail for rape and Jack finally has Dwight out of his life. They're celebrating… and hey, who wouldn't after missing out on such delightful possibilities? So even though we know things are going to stay bad, Wolff wants to leave us on a hopeful note.
Why? Probably to remind us that hope is always there. Jack has been through a wringer, with the evil stepdad and the beatings and the low hum of petty criminal-dom. But there are good things to look forward to and a belief that somehow it's all going to turn out okay. Dwight couldn't break his spirit (even though he broke plenty of other things), and Wolff wants to make sure we remember that when we put the book down. It might not get better, but Jack's a tough little guy and might just have the bounce-back tenacity to get through it.