How does Dave look positively at his abusive past?

The epilogue flashes forward. Dave is standing at the edge of the Pacific Ocean, thinking about how his dark past is behind him. He has tried to let go of his past, and has found that the challenges of his past have made him stronger, given him a new appreciation for life. When he got older, he enlisted in the U.S. Air Force, which gave him a sense of pride and belonging.