How was Eduard Keller a good teacher to Paul Crabbe?

Initially hesitant toward his teacher and his strange ways, Paul learns everything he knows about piano and life from Keller. Keller is initially reserved toward his Paul, but over time, he comes to love and care for him more and more. Keller wants Paul to succeed in both his piano playing and his life. He shares his knowledge and experience with him so Paul can learn from his life. Paul absorbs his teacher's instructions like a sponge, ruining him for any other teacher..... unfortunately, he eventually becomes unteachable.