Sometime I hear this idea that a Guru or a Teacher is not really necessary on the path of Yoga. Be your own Guru. Find the Teacher within. These are common sentiments that are out there in the Yoga Community at large.
The true teachings of Yoga however, have always been passed down through the relationship of Teacher to Student in a line of transmission called parampara. The Guru is absolutely necessary. He or She is there to shine a light on your blind spots.
The Teacher is your road map on the spiritual path. God works through your Guru to help guide you in your practice and eliminate any inner obstacles that can come up along the way. In India they have a saying that "just as you need the flame of an already burning candle to light the wick of another candle; likewise, only an illumined soul can light the inner lamp of another person." Consequently, we need the direct contact of a True Teacher to help enlighten us, to wake us up inside, and get the spark going.
I would say that it is difficult, if not entirely impossible, to see ourselves clearly. As a result, we need the help of a Guru to hold up the mirror and give us clarity so that we can overcome our lower nature and negative qualities. The Guru represents the one's ideal of perfection, and is the pattern from which one wishes to mould oneself after. It is a challenge for the human mind to conceive of an Infinite Formless God, so in the physical form of the Guru, the mind attaches itself more easily and begins to see God within this physical embodiment. Through our reverence, obedience, and willingness to surrender to a True Teacher we are able to connect to the Ultimate Reality that exists everywhere beyond form and time.
On a practical level, the Teacher is able to lift our spirits and helps to keep us motivated in our practice during difficult times along the path. Even the mere presence of one's Teacher can help elevate one’s mood and enhance one's consciousness.
However, the work, the sadhana, the spiritual practice must be done by the student. A Guru cannot bestow a miraculous awakening onto anyone, but He or She is there to show us the door, but as a student, we must do the work, and have the courage and determination to walk through it.
Om Tat Sat