The Spiritual Import of the Mahabharata and the Bhagavadgita : Ch -18.4.

Chapter -18: Fix Your Mind on Me Alone-4.

The reason is purely psychological, which is the essence of the whole matter in contemplation or meditation. Meditation proper is what usually is known as ananya chintana—a thinking deeply, absorption wholly, to the exclusion of any extraneous idea.
This is the basic psychological secret in contemplation or meditation.
The function of the mind at the time of meditation is very important, not the nature of the object.

The purpose of meditation is to so adjust the mind to a particular pattern of thinking, so that it ceases from any distracted attention towards dualistic notions which sustain the ego individuality of a person.
The whole point in meditation is transcendence of thought—overcoming of ego and dissolution of personal consciousness in God-Being.
This can be achieved only when the mind is freed from its attachments to diversity of thought and the multitudinous attention that it usually bestows upon objects of sense.

When the mind is concentrated on any particular ideal—externally a form or internally a concept, whatever it be—what happens is there is a bombarding of the mind by a single thought.
Just as we hear of the bombardment of material particles by scientific methods due to which a tremendous energy is released out of particles of matter merely because of the continuous hammering on them by great forces imposed upon them from outside, the energy of the mind gets released by a continuity of thought which presses upon it so hard that it bursts forth, as it were, and overcomes itself.

There is a self-transcendence of the mind by a repeated hammering over it by thoughts which are continuously maintained to the exclusion of anything else.
Swami Krishnananda
  To be continued  ....