How is loss an important theme in the book?

Loss is something every person will experience in their life, whether it's a family member dying, a friend fading away, the loss of a job or a dream, or something else entirely. Josh not only faces the loss of JB as his best friend (or at least that's how he interprets it) but also the loss of his father to a heart attack. He handles the former with frustration, attempts to hold on, and finally anger and outburst. He learns from this, though, and when Dad passes away he approaches his loss with more thoughtfulness, reflection, and reaching out towards others rather than pushing them away. Alexander doesn't mitigate the pain of Dad's death—it is deeply, deeply sad and affecting—but he does suggest that the family will endure.