The human body is the boat, the instructions of Lord Kṛṣṇa are the favorable winds, and the spiritual master is the navigator. The spiritual master knows well how to adjust the sails to catch the winds favorably and steer the boat to its destination. If, however, one does not take advantage of this opportunity, one wastes the human form of life. Wasting time and life in this way is the same as committing suicide.
Source: Srimad Bhagavadam 4.23.28