2:2 On Death & Duty
Kṛṣṇa smiled reassuringly and began speaking calmly to Arjuna, although right in the middle of two armies on the brink of war:
“You are using words like ‘karma’ and ‘peace’, like a wise and saintly person – but those who really are wise and saintly are never overwhelmed with depression like you are. They know that there is nothing to lament over, neither in life nor in death. [śl. 11]
Arjuna would counter, “How can there be nothing lamentable in death? When a person dies they lose everything and we lose them!” So…
“People never cease to exist,” Krishna explained. “I always existed. You always existed. All these people always existed. And all of us will always exist. 
This and the following two ślokas are extremely important statements in the Gītā. The current śloka establishes two fundamental points:
(1) The physical law of “conservation of energy” applies to the life-force (“soul”).Energy is never lost, it merely transforms. Life-force has always existed and will always exist – although, as we will see in the next śloka, it always changes the form in which it expresses itself.
(2) Life-force maintains integrity throughout all its changes of state. Each individual entity – be it Krishna or you or I – has a distinct identity which has always been intact since before time began, and will always remain intact even when time is wound up and restarted, or exited via liberation. This directly refutes the monistic idea that a liberated soul eventually loses its identity and “merges” into God in such a way that destroys its individual existence.
“Life-force constantly changes its form. Even in this one lifetime it passes from childhood to youth and eventually to old-age; and after this lifetime it similarly transforms into an entirely new body. People who really are wise are not confused about this. 
The law of conservation of energy states that energy is never lost, it only changes form. Krishna explains that the energy called “Life-force” or “soul” constantly changes form, even over the course of a single lifetime. When our current body dies, the life-force continues to change form, this time entering/ creating a new body. This establishes the concept of reincarnation in a scientifically rational manner.
Arjuna is very likely to counter by saying, “Although I accept all of that to be true, there is no denying that the death of my loved ones is a very unpleasant transformation to experience.” Krishna therefore says:
“What is ‘pleasure’? What is ‘displeasure’? They are nothing more than the superficial contact between our senses and desirable or undesirable objects. They are just a sensation, like hot and cold; and they come and go on their own just like winter and summer. You should not make important decisions based on superficial pleasure or displeasure! You should tolerate pleasure and displeasure and always do what you must.  If you base the decisions of your life in this way, you incur no karma at all and become liberated. ”
Arjuna’s inability to fight arose from him realizing that, win or lose, he would suffer great and terrible pains and emotional losses. Krishna’s response is that pleasure and pain is not important to a wise and moral person. A wise and moral person always does what they must, regardless of if it happens to be pleasant or unpleasant. Because they do all their actions without personal motivation, they do not incur karmic responsibility for their deeds, and thus they soon become freed from the disturbing cycle of pleasure and pain and return to a condition of unmitigated constitutional joy.