Bhagavad Gita Chapter 2 Sloka 27 (2.27)

ಜಾತಸ್ಯ ಹಿ ಧ್ರುವೋ ಮೃತ್ಯುರ್ಧ್ರುವಂ ಜನ್ಮ ಮೃತಸ್ಯ ಚ ।
ತಸ್ಮಾದಪರಿಹಾರ್ಯೇSರ್ಥೇ ನ ತ್ವಂ ಶೋಚಿತುಮರ್ಹಸಿ ॥೨೭॥
jātasya hi dhruvo mṛtyurdhruvaṃ janma mṛtasya ca |
tasmādaparihāryeSrthe na tvaṃ śocitumarhasi ||27||
Gist of the sloka:
For one who is born, death is certain. Those who have died, birth is certain. When this is the reality, why would you lament for something which is definite and unchangeable?
Explanation:
Birth/death, happiness/grief are like two sides of a coin. Without night, we will not have sunlight next. One follows the other in certainty.
To lament and fall into grief due to the inevitable events is not appropriate. One should therefore learn to understand and grow out of it.
So is the case of executing one’s duty. One should not fail to execute such duties.
One does not cry due to sunset, resulting in disappearance of the sun. For sun raises next day and none can stop this inevitable event. Similar should be case when our duty calls us.