When I was a youngster, I was handed a firecracker. My experience with it told me that if I was given anything that was lit I was supposed to throw it or I could get hurt. The jerk of a friend of mine, unfortunately, was holding my hand steady, and denied me the freedom to throw said firecracker away. It went off with a loud snap – in my hand, and I was surprised. Not only was it loud, but I still had my fingers. I learned at this time how he had lost the ability to bend his middle finger, and learned a valuable lesson. Limitations are often the most influential factor in the potential of a subject.
I work with a number of teachers, and expect them all to push me further than the previous one. I find myself emulating this with all of my friends and colleagues. What it comes down to is, if I am not being pushed, I am not achieving what I could be and as such, if I am working with people and they are not achieving what they could if I were not to push them hard.
Of course, there is a difference between a teacher being hard on a student and a teacher that is just being destructive to the relationship that could be developed otherwise. There is a fine line between pushing and shoving, and shoving to one person may be gentle coaxing to another.
The moral of the story is to step out of your comfort zone, even if you are already above your current level of instruction, or a teacher. Allow yourself to be pushed and go further than you believed was possible previously. You will always be the better for it.