Bhagavad Gita Verse 27, Chapter 2

jaatasya hi dhruvoo mrityudhruvam janma mritasya cha |
tasmaadaparihaaryerthe na tvam shoochitumarhasi || 27 ||
 
Since one who is born certainly dies, and one who dies certainly is born. Therefore you should not grieve over this inevitable fact.
 
jaatasya : one who is born
hi : since
dhruvaha : certainly
mrityuha : die
janma : born
mritasya : who is dead
cha : and
tasmaad : therefore
aparihaarye:  inevitable
arthe : fact
na : not
tvam : you
shoochitum : grieve
arhasi : should
 
In the last shloka, Shri Krishna told Arjuna: Even if you think that the eternal essence undergoes birth and death, you should still not grieve. He continues the argument in this shloka.
 
The notion that birth eventually results in death, and death eventually results in birth is sometimes difficult for us to accept emotionally, but at the intellectual level, most of us acknowledge and accept it. I remember watching a TV show that showed a time lapse (high speed) video of a rodent's corpse decaying into the soil, and small plants and flowers emerging from the soil shortly thereafter. I thought that it very vividly and visually illustrated the cyclical nature of birth and death.
 
If we look at this example closely, we conclude that the physical body of the animal transformed into the raw material for the body of the flowers and plants. And although we could not see it, we can guess that the eternal essence of the animal "died" and is now "born" as the life force that sustains the plants and flowers.
 
Therefore we would not grieve for death the animal's body, nor for the death of the life force in it, because both were born again after they died. Similarly, Shri Krishna wanted Arjuna not to grieve for the imminent death of his kinsmen.