"When you accept a person as your friend, you mean you accept him as your equal - a friend is equal to you. Yes, you are friends to me because I see that you are just equal to me, there is no difference. But if you see me as an equal to you, then your growth will stop." - Osho
Friends cannot teach friends because they view one another as equals. In order for true learning to occur, you must learn from a Master, not a friend. But people with big egos cannot accept anyone as a Master, they want to turn everyone into friends, into equals. When you accept everyone as equals, learning stops. For the benefit of the disciple, the Master cannot encourage the disciple to view him as a friend, otherwise the the growth of the disciple will be stunted.