In plain words
We grieve over what does not deserve grief. Why? Taking the upādhi for our very form, deluding ourselves that the body is our all in life, we strive to do every deed for the sake of this body made of the five elements; and what this frustrated hope returns to us is the root cause of our sorrows. Kṛṣṇa said the same again in the eighteenth chapter. There is no need for you to grieve. What is the cause of this grief? All objects, all beings, all deeds: they are you! Unable to recognise your true nature, not knowing the truth that the divine Ātma-principle that is ‘I’ pervades everywhere: that is the root cause of grief.
In the Bhagavad Gītā, Dhṛtarāṣṭra begins with the question of what his sons and the sons of Pāṇḍu are doing on the battlefield. Dhṛtarāṣṭra is blind. What does blind mean? One is not blind merely because the eyes of flesh are gone. He who believes the One to be many and follows that belief: he is blind. He who takes as his own what is not his: he is blind! He who tries to live his life in the feeling ‘This body is I’: he is blind. He who falls prey to attachment and aversion and is sunk in ignorance: he is blind. Though what exists is One, he sees it in many forms. That is the root cause of blindness.
Though they have eyes, they are blind; Your auspicious form they could not see.
Understand the one Ātma, and grief has no room
Man has eyes, but he takes everything they see to be real. All of these are bound to change some day! They are like passing clouds. Not one of them is lasting. If man recognises this truth, there is no room at all for grief. If the one Ātma is understood, there is no room for grief.