Adam Troy Castro penned a wonderful review that addresses the novel’s why and ending without really spoiling either.
“The Question Worth Answering
Many, many years ago, your devoted servant was more obviously still learning this sometimes-disreputable craft (he still is, hopes never to stop, and avers that any writer who says otherwise is likely not worth reading). A workshop participant provided him with the following comment, written in red Bic Pen in the margin of his current horror story (one that eventually got published, despite being not very good), beside a gratuitous, but particularly loving description of shotgun-scattered brain matter slowly sliding down a tenement wall.
She wrote, “Is this trip necessary?””
Paul Tremblay (the online version!) 
Pretty cool review. I’m sure it made your day.