Purport

First Bhagvan Shri Krishna explained Arjuna, about truth of living being. Bhagvan Shri Krishna explained Arjuna that livng being is not this body, but soul, and soul is eternal. It is is birth- less, all pervading and primeval and cannot be harmed or hurt by any physical means. Birth and death is nothing but the physical change. Soul takes this body same way a human takes clothes over his body. Therefore a wise should not grieve for the physical process of birth and death. This is the eternal fact.

But then Bhagvan Shri Krishna explains that even if it is difficult for a person to understand the soul, and he considers living being as physical being made of body, still there is no reason to grieve. Because anybody who has taken birth has to die, It is just a matter of time, but it is inevitable. Again a wise person should not grieve for something which is inevitable.