Everything that Finny does and says makes Gene jealous of him during summer school. The boys are at a point in their lives where they are self-conscious and it is natural to have those feelings. Because Gene has feelings of jealousy, the tension builds until Finny says that he wouldn't like to see Gene succeed academically and be the first in their class. This "proves" to Gene that Finny isn't a true friend and thus resents him. Gene doesn't respond to Finny when Finny claims his as his best pal, but we only get the adult's perspective looking back on that event and we learn that he wished he had said something back. However, it would seem with all of the evidence of Gene's jealousy that he didn't feel the same way and that is why he said nothing. In fact, Finny says this to Gene before the Tree-bouncing incident, so at the time Gene must have still felt insecure with Finny. Regretfully, adult Gene wishes after the fact that he would have responded better.