Arjuna’s reaction assumes the primacy and reality of the physical life of the body, and thereby treats the deaths that are about to occur as being a tragedy of the highest order. Sri Krishna needs to both respond to the normal standpoint that accepts this premise, as well as introduce the higher truth of the soul as the controlling fact, and thereby reorient Arjuna’s viewpoint to one that is not based on the illusion of the body.
Just as, in our normal viewpoint, we talk about the sunrise and sunset, when it fact it is the earth moving around the sun, rather than the sun moving around the earth, we need to recognise that our normal view of life and the physical reality of the body is similarly skewed by treating the body as primary and the soul as a consequence, rather than recognising that it is the soul that takes birth in forms, and that it will change forms and take a new birth after the death of the present body.
Sri Aurobindo explains: “The enlightened man does not mourn either for the living or the dead, for he knows that suffering and death are merely incidents in the history of the soul. The soul, not the body, is the reality. All these kings of men for whose approaching death he mourns, have lived before, they will live again in the human body; for as the soul passes physically through childhood and youth and age, so it passes on to the changing of the body.”
“The calm and wise mind, … the thinker who looks upon life steadily….is not deceived by material appearances; he does not allow the clamour of his blood and his nerves and his heart to cloud his judgment or to contradict his knowledge. He looks beyond the apparent facts of the life of the body and senses to the real fact of his being and rises beyond the emotional and physical desires of the ignorant nature to the true and only aim of the human existence.”
Sri Aurobindo, Essays on the Gita, First Series, Chapter 7, The Creed of the Aryan Fighter, pp. 55-56,