Before I answer this question, I will give a small example. With this you will understand with more clarity. Suppose you need a LIC (Life insurance) policy. There are many ways of buying it. You can go personally to the counter and get it or you can approach an agent. If you go through an agent, then he can guide you better. He would explain in detail about the various types of policies, the returns of investment on each of them as well as the one best suited to your needs. In addition, he will make sure your money is deposited properly into your account. Can you get all this services if you go personally to the counter?
In the recitation of the mantra, clarity, inhalation, exhalation, proper splitting of the words, arohana (ascending sequence of notes), avarohana (descending sequence of notes) etc. are extremely important. When the mantra is chanted exactly as per the stipulated pattern it generates a tremendous energy in the person. This chanting procedure cannot be learnt from books. That is why a mantra has to be taken from a Guru.
There is another reason too. Didn’t I say that the repetition generates a type of energy? It is important that this energy is not misused; it needs to be channeled properly. If this is misused, the sin would fall on the Guru who initiated the person. That is why the Guru assesses the person thoroughly before initiating him.
The third reason is the most important one. While initiating, the Guru transfers some of his energy to his student! Just as a mother shares her body, blood etc. for a baby’s growth in her womb, a Guru shares his power of penance (tapas shakti), his spiritual knowledge (Jnana) with his disciple. Now you understand why it is necessary to approach a Guru for mantra!