Bhagavad Gita...grieving for the dead

The blessed Lord said,
'You grieve for those who should not be grieved for;
yet you speak wise words.
Neither for the dead nor those not dead do the wise grieve.
Never was there a time when I did not exist
nor you nor these lords of men.
Neither will there be a time when we shall not exist;
we all exist from now on.
As the soul experiences in this body
childhood, youth, and old age,
so also it acquires another body;
the sage in this is not deluded.

"'Material sensations, Kaunteya,
causing cold, heat, pleasure, pain,
coming and going are impermanent;
you must endure them, Bharata.
The person whom these do not trouble, powerful person,
pain and pleasure being equal to the sage,
he is ready for immortality.

"'The existence of the unreal is not found;
the non-existence of the real is not found.
The certainty of both of these has been seen
by the seers of essence.
Know that indestructible essence
by which all this is pervaded.
No one is able to cause the destruction of the imperishable.
These bodies have an end;
it is said of the indestructible, infinite soul
that it is eternal.
Therefore, fight, Bharata!

"'Whoever believes this the killer
and whoever thinks this the killed,
they both do not understand;
this does not kill and is not killed.
Neither is it born nor does it die at any time,
nor having been, will this again not be.
Unborn, eternal, perpetual this ancient being
is not killed with the killing of the body.

"'Whoever knows this, the indestructible,
the eternal, the unborn, the imperishable,
how does this person, Partha, cause the killing of anyone?
Whom does one kill?
As a person abandoning worn-out clothes takes new ones,
so abandoning worn-out bodies the soul enters new ones.
Weapons do not cut this nor does fire burn this,
and waters cannot wet this nor can wind dry it.
Not pierced this, not burned this, not wetted nor dried,
eternal, all-pervading, stable,
immovable is this everlasting.
Unmanifest this, it is said.

"'Therefore knowing this you should not mourn.
And if you think this is eternally born or eternally dying,
even then, you mighty armed, you should not mourn this.
Death is certain for the born,
and birth is certain for the dead.
Therefore you should not mourn over the inevitable.

"'Beings have unmanifest beginnings,
manifest middles, Bharata, unmanifest ends again.
What complaint is there?
Marvelously someone sees this,
and marvelously another thus tells,
and marvelously another hears this,
but even having heard no one knows this.
This embodied soul is eternally inviolable
in the body of all, Bharata.
Therefore you should not mourn for any being.