Growing Up
Josh and JB are at the crucial age of passing from being children to young adults. This comes with numerous confusing, exhilarating, and scary experiences: girlfriends for the first time, losing friends and loved ones, making serious mistakes, forming one's identity, understanding the value of loyalty and the pain of loss, and more. Sometimes, the boys behave in ways they don't quite get themselves—for example, Josh does not know how he got so "churlish" when it came to JB—and other times, they are deeply introspective and can analyze why they do or feel a certain way. Growing up is thus a terrifying and magical time, and Alexander manages to convey it with empathy and nuance.
Family
Family is one of the most important themes in the text. In this novel, the family unit is a supportive, uplifting, inspiring, ordering, meaning-providing, and sustaining entity. Mom and Dad are thoughtful, engaged parents who want the best for and encourage the best in their children. Josh and JB are loving sons; sometimes they display some negative traits characteristic of a young teenager, but they ultimately have honest and healthy relationships with their parents. The family isn't perfect, though: when characters let the family down (e.g., Josh hurting JB, Dad refusing to go to the doctor), it causes shockwaves throughout the unit. Josh often wonders if their family is "falling apart" because of what is going on, and while, of course, it isn't, it is definitely enduring some attacks on its unity and health.
Inheritance
Josh and JB are proud to be the sons of Chuck "Da Man" Bell. They've inherited his prowess on the court and are slated to go just as far, if not further, than their father did in their basketball careers. They embody their father's commitment, enthusiasm, arrogance, and pride in their playing, and it's almost as if they didn't have any choice but to inherit these things from Chuck given how much he emphasized them when the boys were babies. However, inheritance is more complicated than that. The boys realize that they are also inheriting Dad's proclivity to hypertension, and perhaps, if they're not careful, his willful ignorance and antipathy towards getting the care he needs. Inheritance then is something to be thankful for but also to be aware of how it may limit one's freedom and individuality.
Identity
Part of growing up is learning who you are, and Josh is faced with determining what kind of identity he wants to shape for himself. He's clearly a talented basketball player but a little arrogant. He values family tremendously and does not like any change in their dynamic. In fact, he does not particularly care for change at all, enjoying the status quo in which he is generally happy and successful. As the changes to his relationships come whether he likes it or not, he is confronted with aspects of his identity that he is not quite so comfortable with: his jealousy, his resentment, and his bursts of anger. Alexander shows that if we undertake the challenge, we can understand why we are the way we are and how to work at bettering parts of ourselves that are not ideal.
The Game of Life
Alexander uses basketball as an allegory for the game of life, suggesting that the lessons one learns on the court can apply to life off the court. He does this through Dad's "Basketball Rules" as well as aspects of the plot in general. Players on the court make mistakes or do incredible things, work together, sometimes win and sometimes lose, have to work hard and constantly be learning, and have to confront their assumptions about themselves. These all apply to life off the court as well, and Josh is the reader's guide for how this plays out.
Atonement
One of the central lessons of the novel is that if you make a mistake, you need to atone for it. Josh could not control his anger and lashed out at JB in an aggressive way. Naturally, JB did not want to forgive his brother right away and forced Josh to consider his mistake. Josh atoned through numerous ways, such as being patient, writing JB a letter, spending time by himself to understand why he behaved as he did, giving JB study materials for the test, and being there for him when their Dad got sick. Josh admirably did not lose his patience with JB or push him too hard, which is an excellent thing for readers to acknowledge. It is easy to wish someone whom we've wronged would just get over it, but oftentimes we need to be patient and bear the situation out. That is what Josh does, and readers can learn from his hard but necessary lesson.
Loss
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.