A huge log of solid timber floats on the water and can carry men and animals across the bank, even in rising waters. But a piece of worthless ordinary wood sinks as soon as one steps on it and drowns the person. Therefore, in every age God incarnates Himself as the Guru, to teach humanity. Satchidananda alone is the Guru. A Guru looks into the nature of the people before making them his disciples.
All men look alike but have different natures.
Some have an excess of sattva, others excess of rajas, and still others an excess of tamas. Accordingly, Guru gives differing instructions for spiritual aspirants. It is extremely difficult to become a teacher. A man can teach only if God reveals Himself to him and gives command. Then his words are very forceful.