In case any of us might still be wondering if such harshness is really necessary to reach enlightenment, let us take the example of a vessel that is encrusted with rust. The rust on the metal container is so rough that trying to remove it with a soft cloth and gentle hand, we would not be able to remove any rust at all. The more effective way to remove the rust would be to find another very rough substance even rougher than the rust. If we rub the container with this, then the rust can be removed. Similarly, the negative karma obscuring the true nature of mind cannot be removed by softness or gentleness, nor can the achievement of realization occur if a teacher is overly kind.
Harshness is needed.
If a teacher allows his students to be lazy and is too soft-hearted, the students will be unable to uncover and purify their mental stains. Harshness and roughness on the part of the teacher are essential. After Naropa reached the stage of realization equal to his teacher, Tilopa assured him that they were inseparable. They had never been separate in the past, were not separate in the present, and would never be separate in the future. Renowned for their realization, wisdom, and profound learning, Tilopa and Naropa turned the wheel of Dharma uncountable times in all directions.