Sri Chinmoy: No one is forcing you to have a Master, but it certainly is advisable. You know that there is a goal, and you want to reach the goal. If you are wise, you accept help from someone who can show you the easiest, safest, and most effective path to that goal. If you want to take hundreds and thousands of years to realise God, a spiritual Master is not necessary. But if you want to reach the Goal the fastest way, then certainly he is a necessity.
If you want to become a doctor, you go to school and study with doctors. If you do not study with a doctor, you will not operate on someone, because there you are dealing with human life. The Guru is like a spiritual doctor. He will operate on your fear, doubt and jealousy. First you doubt the existence of the Vast, the Unknown. Then you are afraid of the Vast because you feel that it is not a part of you and that it will engulf you. And then you are jealous of the Vast, because you feel that it is all illumination, fulfilment and perfection, whereas your life is all obscurity, frustration and imperfection. So before you can operate on others, first you have to study with a spiritual doctor, the spiritual Master. He will cure you and then teach you how to cure others.