There are many incidents within the story that reflect this approach.
When Gene and Finny are studying before the fall, Gene considers that Finny is actively trying to distract him from his studies because he can't stand to have Gene be better overall.
I was more and more certainly becoming the best student in the school; Phineas was without question the best athlete, so in that way we were even. But while he was a very poor student I was a pretty good athlete, and when everything was thrown into the scales they would in the end tilt definitely toward me.
The insights into Finny's perceptions and thoughts gained as they talked after the fall, and especially after Finny returns to school, were growth moments for Gene in self-awareness and in maturity of attitude toward Finny and the others.
When Leper joins the ski patrol, he anticipates spending the war in a clean white world of snow and skiing. The boys left behind deal with his enlistment by placing him in the center of every action reported. It is only later that Gene becomes aware of the tragic suffering Leper encounters while in the military.
Fear seized my stomach like a cramp. I didn't care what I said to him now; it was myself I was worried about. For if Leper was psycho it was the army which had done it to him, and I and all of us were on the brink of the army.