A bit of Intro:
Bhagwad Geeta is a conversation between Lord Krishna and Arjun in the middle of a battlefield at Kurukshetra, now in Haryana. The war in Mahabharat is described as a war of dharma, meaning a war of righteousness. The war wasn’t fought out of bitterness but because it was inevitable. The war took place sometime around 3150 BCE (about 5000 years ago). In the war Arjun is supposed to be a leading warrior. However when the war starts he is in a way shocked to see that he would be fighting against his cousins, elder relatives and teachers. He refuses to fight saying that his dharma (righteousness) forbids him from killing them. He says a lot of stuff to Krishna that he would give up arms and become a hermit and all that. He is confused. The first chapter and the first few verses generally involve Arjun crying. Usually the trick to reading the geeta is to start from the 2nd chapter verse 11.
asocyan anvasocas tvam
prajna-vadams ca bhasase
gatasun agatasums ca
The Blessed Lord said
You have been mourning for those who should not be mourned for. Yet you speak words of wisdom. The truly wise grieve neither for the living nor for the dead.
Whatever Krishna says starts from the basic assumption that Arjun knows that life is immortal. This is the basic assumption of any spiritual text of ancient India. Life is immortal. We just die and are born continuously over the lifetimes. The goal of life is to understand our true nature. The enlightened people are those who know what our true nature is and according to them we are just the soul, the atma, bramhan, love. All these words are synonymous. When we actually understand this then we are liberated from the cycle of life and death. But until then, we have no choice but to keep coming back. And hence Krishna tells Arjun that there is no point in crying for the living or the dead. One must also remember that while we read this we are all Arjun. Many times we find ourselves crying for our fate or for the fate of our near ones. “Why does this happen to me? What wrong did I do?” One must remember that we are all living according to our karma. Krishna in this mocks Arjun saying that you talk like a wise man in saying that you cannot fight with your relatives and that it is against your dharma but then you forget what is the actual truth. The actual truth is that those, for whom you are crying for, will not die.