Why we need a spiritual teacher ?- Extract from works of Swami Vivekananda

Every soul is destined to be perfect, and every being, in the end, will attain to that state. Whatever we are now, is the result of whatever we have been, or thought in the past, and whatever we shall be in future will be the result of what we do or think now. But this does not preclude our receiving help from outside; the possibilities of the soul are always quickened by some help from outside. In fact in the vast majority of cases in the world, help from outside is absolutely necessary. This quickening impulse, which comes from outside, cannot be received from books; the soul can receive impulse from another soul , and from nothing else. We may study books all our lives,  we may become very intellectual, but in the end we find we have not developed at all spiritually. 

It does not follow that a high order of intellectual development always shows an equivalent development of the spiritual side of man; on the other hand we find cases almost every day where the intellect have become very highly developed at the expense of the spirit. This is the reason that many of us can speak most wonderfully on spiritual subjects, but when the time of action comes, we find ourselves so woefully deficient. Many times acclaimed Gurus, with huge fan following by their mesmerising and intellectual  talks, behave in a manner that is not worthy of a developed of soul or spirituality.

The soul from which this impulse comes is called Guru or the teacher. In order to convey this impulse the Guru must possess the power of transmitting it and the disciple must be fit to receive it. The seed must be a living seed, and the field must be ready ploughed. When both these conditions are fulfilled, a wonderful growth of development of soul takes place.

The precondition for this to happen is that disciple must have a strong and sustained urge to develop and not just a momentary feeling driven by some mishap in life and the prerequisites are;

1. Purity 
2. A real and lasting thirst after knowledge
3. Perseverance

Guru must be the right one fulfilling the necessary conditions as follow;

1. He must have the knowledge of scriptures and understands and practices the spirit of the scriptures - Vedas, Bible, Koran...
2. He must be sinless by his action and conduct
3. His motive is pure for disciple's development and not any other motive, fame or money.

Only if the above two set of conditions are met, the real progress begins.