Bhagavad Gita

ಭಗವಾನುವಾಚ ।
ಅಶೋಚ್ಯಾನನ್ವಶೋಚಸ್ತ್ವಂ ಪ್ರಜ್ಞಾವಾದಾಂಶ್ಚ ಭಾಷಸೇ |
ಗತಾಸೂನಗತಾಸೂಂಶ್ಚ ನಾನುಶೋಚಂತಿ ಪಂಡಿತಾಃ ॥೧೧॥
bhagavānuvāca |
aśocyānanvaśocastvaṃ prajñāvādāṃśca bhāṣase |
gatāsūnagatāsūṃśca nānuśocaṃti paṃḍitāḥ ||11||

Gist of the sloka:
Lord spoke: You are crying for them for whose sake one should not cry. You are speaking irresponsibly with a belief as though you are a knowledgeable and a wise person. Those who are wise, never lament for living or dead.
Lord Krishna puts a verbal mirror to show Arjuna how foolish are his thoughts. He also brings down Arjuna’s ego by indicating he is not a knowledgeable person either.
The root cause of Arjuna’s grief is the fact that some seniors would die, some evil people might survive and therefore fear. This fear is due to lack of correct knowledge. Lord Krishna therefore sets right the knowledge factor.
Lord Krishna therefore indicates that the words spoken by Arjuna are not the same words spoken by a knowledgeable person. Wise men would have very clearly understood the concept of death, birth and its relationship with soul. He clearly indicates to Arjuna that Arjuna is speaking, assuming his words are the universal truth but when in fact, they are actually not.
Lord Krishna therefore educates Arjuna on the truth which is universal and never changing. Arjuna knowledge was limited to this planet whereas the Lord was giving him the different perspective which is universal and eternal.
Question might arise, should one not fall into grief if seniors like Bheeshmacharya and Dronacharya were to die. To decide on the right way, we first need to understand what exactly is death and birth. Once we too understand the universal truth about this aspect, we too will understand why Arjuna’s words were irrelevant.
Individual soul is alive since time immemorial [Sanathana]. Its birth did not begin with being present in the body nor its death after it leaves this body. It is similar to growing out of our clothes and dressing up with new ones. It is the cloth which was discarded not us. We don’t mourn the loss of the cloth and neither should we for loss of this ‘case/cage’ called body. One who has understood this, would have no fear about death.
Death is not fearsome. Birth is inevitable [due our prarabdha karmas]. Soul has no death. Soul has existed from beginning and will continue to do so.
Whereas the gross body made out of perishable materials is meant to deteriorate and destroy itself. Such an event when the body merges back with the environment/prakruthi is death [of the body]. One should therefore should not cry for what is inevitable in the cycle of prakruthi. Neither should one cry for soul as its always present and alive.