Jealousy and self-deception
Gene's jealousy of Finny is real, but Gene hides it from himself by constructing a false theory of rivalry. The novel shows how envy doesn't just damage other people. It distorts the person who carries it, making them see threats that don't exist and miss the damage they're actually doing.
Guilt and the cost of avoiding it
After the fall, Gene lives with guilt he can't fully confess. Finny won't hear the confession. Gene can't force the truth out. That avoidance doesn't make the guilt smaller. It makes it heavier, and it follows Gene all the way into adulthood.
The loss of innocence
The summer term at Devon feels protected from the war and from adult consequences. The fall from the tree ends that. The winter term is colder, more rule-bound, and closer to the real world. The novel tracks exactly how that shift happens and what it costs.
War, internal and external
The novel puts a private war between two friends alongside a world war happening outside the school gates. Gene eventually argues that the external war is just the internal one scaled up. Fear and hatred inside individuals are what make wars possible.
Identity and competition
Gene defines himself partly against Finny. When Finny is better at something, Gene feels diminished. After the accident, Gene literally wears Finny's clothes and feels briefly like he has become Finny. The novel asks how much of who we are is built on comparison to the people around us.