Every intellectual matter is related to power and forcefulness, just as fire is powerful and forceful. It shies away from nothing, and has no problem injecting itself wherever it wants or needs to be. This is comparable to fire, which has the ability to consume everything in its path, turning it in to part of the fire, and shying away from nothing. The Torah, which is like fire, was given to the Jewish people due to their boldness and forcefulness. In order to receive an object, the recipient must be able to have a relationship with the object he is receiving.
Man can receive a law with the power of fire.
But if fire of the teacher is added to the fire of the Torah, then man is being asked to receive something which is completely fire, which is beyond his human capacity. There is no question that a teacher must demand discipline from his students, both intellectual as well as behavioral discipline. But being Overly strict would be caused by the teacher's insecurity, leading him to exert his ego. This should be contrasted to appropriate strictness, which would result from the teacher's desire for the true well-being of the student. If the only fire is that of the Torah, man, if he is strong and bold, can receive that, since its nature is one of compelling transcendent truth and wisdom.