Chapter 2,Verse 27

jatasya hi dhruvo mruthyuhu
dhruvam janma mrutasya cha
tasmad apariharthertheye
na tvam shochitum arhasi

Now.Krishna gives a reason as to why Arjuna should not grieve over the death of his relatives.He says that for a being that is born,death is certain.And for one which has died,birth is certain.This is an undeniable fact.And one has no control over this.Hence,one must learn to accept what is inevitable.Intelligence is preparing the mind for situations over which one has no choice.The Mahabharatha is an inevitable event pending to happen,and Arjuna has no contol over it.There would be lives lost on both sides.He would have to part with his near and dear ones.But,he must accept this fact,and move on.By resisting to do so,one will only encounter sorrow.If accepted,the pain is halved.If resisted,the pain is doubled.
Now comes the question of how to develop this attitude of accepting the inevitable without resistance.To begin with,one must keep meditating/saying to oneself that he/she is ready to accept the inevitable.On continually practising it,it would gradually get imbibed in our hearts.Our facial expressions and body language would then start to reflect it.Others would then be able to see a smile on our faces at all times.Krishna asks Arjuna to do the same.According to our shastras,the mind and the brain are differnt.The brain forms part of the visible body,whereas the mind,the invsible.The brain perishes when one dies,whereas the mind continues.But he tells Arjuna that,even if he refuses to believe in this,he must still not grieve over the death of mortals,be it Bheeshma or Drona.
Death is inevitable.Modern science can extend life and delay death,but can never overcome it.We must all accept this fact.By understanding that one is the atma & not the physical body,two purposes are fulfilled.
1.It gives inner peace as I know that I am the atma,and that I have no destruction.This way,the innate urge for immortality is fulfilled.
2.I will,hence,be able to accept the mortality of the human body.

There is a very useful prayer that we all can use everyday-

"Oh Lord,give me the serenity to accept what cannot be changed.
Oh Lord,give me the courage to change what can be changed.
Oh LOrd,give me the ability to differentiate between the two"