Keller’s concerns with Paul are based on much more than music.

Much of the novel focuses on the relationship between the student (Paul) and the teacher (Keller) and how the two affect one another. Initially hesitant toward the old man and his strange ways, Paul ends up learning so much from Keller about both piano and life. The teacher is initially reserved toward his student but grows to love and care for him more and more. He wants to see his student succeed in both piano and in life, and desires to share his knowledge with him so that he will learn from Keller's experiences.This is, at heart, a coming of age story. Keller realizes that Paul is undergoing the pain of adolescence as well as becoming a reflective human being. I'd check out the GradeSaver theme section (link below) as they relate directly with your question.

http://www.gradesaver.com/maestro/study-guide/major-themes/