We all are searching for a melody in this discontent life. Those who learn music, they seek the help of one who can harmonize different tunes, (or) those who lay faith in science, they look for a teacher who can explain science to them-----likewise, in the case of deeksha or initiation. This is why shiksha and deeksha, teaching and initiation are one and the same thing.Matters about which we are inexperienced---or ignorant---to remove that ignorance ---we need the guru.Those who command our trust and respect, or those who can help to dispel our ignorance---and thereby instil faith and reverence within us----we need the guru for this purpose.
Thus Rabindranath has said:

"Surer guru, dao go surer deeksha, Mora surer kangal,ei amader bheeksha."
(Guru of melodies,initiate us to music we are pining for music;this is our prayers)

If we can sense this pining, this intense desire within us to make this rhythmless life fall into perfect rhythm, then we need mantra. The very term 'mantra' means sound. Where the sound takes a rhythmic sound. The origin of all mantra is the Omkar ---"-a-u-m kar". It is from the Omkar that the universe is created. Whether it is called: Satva(purity), Rajas(passion), Tamas(ignorance), or we call it: Awakening, dreaming, slumbering, or we call it: creation, preservation,destruction---all these aspects are encased within the "a-u-m kar". This omkar is the most perfect rhythm or synthesized sound, and synthesized sound itself is Brahma. Naam(name), roop(form), asti(being), bhati(glory) and kriya(action)----- these five form the universe. Of these, we can easily follow asti, bhati and kriya, but naam resides the naami or the being. Within the mantra lives the Chaitanya or the consciousness. When we worship an image, then we recognize the being or the consciousness within the image and instill the life in the image. (Then) we do not see the clay but in that clay image we behold the Divine Being. That is why, in this world too---- which appears materialistic, coarse and which we accept as something mutable----- within the outerlayers of this world too, is an eternal spiritual self, which has been described by the theologians as:

"AKHANDAMANDALAKARAM VYAPTAM JENA CHARACHARAM TAT PADAM DARSHITAM JENA TASMAI SHRI GURABE NAMAH"

Until the time we feel deep within ourselves a pining, deep desire, a thirst for the divine music, till then we try to search for peace in our meaningless existence in this unsubstantial world, but we never find it. It is only when we would realize that even within this temporal world, there is a quest for immortality, them----those who have tasted this immortality or have taken the path leading to it----we accept them with trust and reverence. We feel---they can guide us to the right path.

The guru-shishya (initiator-initiated) relationship has continued since antiquity. But in reality there is no difference between the guru and the disciple. This is because the divine consciousness that lives within the disciples, is the same that dwells within the guru too. But when I cannot realize the divine consciousness on my own, then I have to take the help of some external source who can make the realization possible.
So it is said:

"MANDAKINI DHARA, TUSHAR SHUBRA TARA JE SURETE KANE KANE HOLO DEEKSHA"
(Which means---The flow of mandakini, the snow-white flow, which flows the music, which whispered initiation into your ears.)

In this world we see some fine flowing forces that leave an imprint on us. The music of the Mandakini is flowing by but we cannot capture the inner melody. However, there are those who have absorbed an this music. (They are like) those who have assimilated the mantra and are continously striving to realize the divine consciousness inherent in the mantra, (they are) those who, in this rhythmless world are seeking the rhythm of cosmic melody, if we surrender to them, then we will be able to play on the music, the resonance that is suppressed within us. That is why the guru also must have that spiritual strength, because only then can he direct spiritual strength to his disciple.

If we have respect and faith, them, the conscious self---whom we revere and trust ---(embodied in) him who has taken. The right path or is firmly rooted there (is the one) to whom we should surrender. Only then the music talent within us can be awakened.
So it is said:

"TOR BHITORE JAGIYA JE, TAHARE BANDHANE RAKHILI BANDHIYA ALOR PIYASI SE JE, GUMORI UTHICHE KANDIYA"
(The awakened self within you, is tied down by you it thirsts for the radiance and cries silently)

This restless self is constantly striving to express itself, but since we do not get the right support or the favourable ambience in the external world, we are not able to awake or free our self.
So Kabir sahib has said:

"Tera sai toojhme, yeh jhopri tera baas Kasturi mrig jaisa phirti dhoondte ghaas."
(The lord is within you and you live in this hut like the mask deer. That roams about searching for mere grass)

The fragrance emanates from the navel of the mask deer while it looks for the source of this fragrance all over the world. It may die, never realizing the real source (is in its own body). Similarly the peace and happiness that we are searching for, are not external objects, they are to be felt within our own heart.

Manifestation of the Divine is in every object, similar to the Ganga that flows without making a difference (between one another). If we have no vessel to carry (Ganga's) water, then they're no collection of water. If the vessel is small, then less water can be collected. Just as the Ganga is impartial, so is God, who lives within the inner most recesses of every heart.

We do not know the identity of God.But he has said---where ever you find beauty, find attraction and feel a sense of purification in the presence of one, accept him as your guru. WHO IS GURU? He is a purified container portion of the eternal beauty is within him. Since we are always yearning for that (supreme) manifestation that joy and that beauty, (therefore) the container in which these qualities are found, we accept him as our guru.

In this world it dwells that indivisible entity (Akhandamandalakar) that is eternally manifested. If we try to reach out to him and consider that to be the only worthwhile aim of life, then, we would realize that the supreme one lives within any container that we hold on to. That is why, they say, that the relationship between the guru and the disciple continues through birth and rebirth.

There is necessity for the Guru.Exactly what this necessity cannot be known through arguments from out side. It is something to be realized through the inner experience. How ever, there must be something spiritual in him, because the scriptures have revealed such experiences. To gain entry into the indefinable inner region, one needs guru. It is through the guru, that the mantras realized by the Rishis can be planted in the disciple's heart. If the seed is good, then, through continual perseverance (sadhna) the result can be achieved more easily and if the seed is fake or poor in quality, then, then, no matter how much one may persevere---it will not yield any fruit. If we take the mantra from a book, then to mistakes can be made. But if we can take within us the mantra, which has already been graced by passing through a purified container, then with little effort, we can have total achievement. (So) it is necessary to have the mantra illuminated through one (the guru) whose mortal form houses an illuminated consciousness.
That is why it is said:

"Guru Brahma, Gurur Vishnu, Gurudeva Maheshwarah Guru sakshat param Brahma"
(Guru is Brahma, Guru is Vishnu, and Guru is maheshwar Guru is the supreme Brahma)

Those who have been able to accept the Guru as the (physical) presence of the supreme Brahma, they will see in the guru the manifestation of the illuminated consciousness. To have this vision it is necessary to have reverence and faith. Once this is achieved, then---

"Jotro jotro netro he-re, totre krisno sphoorti Stabar jangome he-re Na he-re Anya moorti"

Guided by this vision, we realize that the supreme one resides in every object. The object only gives him a physical form. So, "nomo Shri Gurumoortaye edam dakshinammoortim." Who is this Dakshinamoortim (the blessed one)? He is one who exists in the illuminated consciousness of the entire universe. He is THE GURU.

"Urustha Mouno vGuakhyanam shishvastu chhinnasanshayan"

Such as the relationship between the guru and the disciple that the guru remains speech less while meditating on God, and the disciple--- if he is able to connect his own mind with that of the guru---then all doubts are dispelled and his consciousness is enlarged that is why there must be a link between the two minds. There are Gurus to (take care of) the five aspects of our existence----- "pancha prakar guru"--for the mind (manomoy), for the physical (deha moy or annamoy), for the life (pranomy), for knowledge (vigyanomoy) and for the ecstatic self (anondomoy). The guru who rules over our physical self is also the one who instills the spiritual knowledge. He is the guru for (total)consciousness or the Sadguru.If we can take shelter in his grace with reverence and faith,then the slumbering consciousness within our selves will be awaken.