The sitting motionless posture is the natural posture for concentrated meditation – walking and standing are active conditions. It is only when one has gained the enduring rest and passivity of the consciousness that it is easy to concentrate and receive when walking or doing anything. A fundamental passive condition of the consciousness gathered into itself is the proper poise for concentration and a seated gathered immobility in the body is the best position for that. It can be done also lying down, but that position is too passive, tending to be inert rather than gathered. This is the reason why yogis always sit in an asana. One can accustom oneself to meditate walking, standing, lying but sitting is the first natural position.
Growing within – The psychology of Inner Development
(Compilation of the works of Sri Aurobindo)