I agree with the above post. Off the back of the idea about "singular conceptions of good," Junior learns that the face of friendship is not always one of laughter and good times. For the majority of the novel, he and Rowdy are at odds because Rowdy feels like Junior is a traitor. Junior tries to regain Rowdy's friendship and does not really understand the complicated feelings that Rowdy has over Junior's going to Reardan. At the end of the novel, the two boys play basketball "without keeping score" which symbolizes the growth that their relationship has had--they no longer let the element of competition stand between them. Junior learns that a friendship may grow through struggle and conflict.
As with any question of this nature, there will be considerable debate. I think that embracing the divergence of thought will allow a better answer or discussion to emerge. In my mind, I would say that Junior's most important lesson is that human consciousness is fluid enough to embrace different conceptions of self. Junior is poised between two competing communities in terms of the Native American community into which he is born and the White one in which he goes to school. Through the different experiences the Junior has in both worlds, he understands that human beings are more than singular conceptions of the good. His father might be lacking in being a source of support for Junior, but he does care for him. The reservation might be a source of pain, but it is a part of Junior's identity and cannot be negated. Junior is a Spokane Indian, but he is also an athlete, an artist, a thinker, and someone who is able to wear different hats in constructing his own identity.