Bhagavadgeeta Verse 2.13
दॆहिनः अस्मिन् यथा दॆहॆ कौमारं यौवनं जरा
तथा दॆहांतर प्राप्तिः धीरः तत्र न मुह्यति ॥ २.१३ ॥
ದೇಹಿನಃ ಅಸ್ಮಿನ್ ಯಥಾ ದೇಹೇ ಕೌಮಾರಂ ಯೌವನಂ ಜರಾ
ತಥಾ ದೇಹಾಂತರ ಪ್ರಾಪ್ತಿಃ ಧೀರಃ ತತ್ರ ನ ಮುಹ್ಯತಿ || ೨.೧೩ ||
dEhinaH asmin yathA dEhE kaumAraM yauvanaM jarA
tathA dEhAMtara prAptiH dheeraH tatra na muhyati || 2.13 ||
Just as childhood, youth and old age befall the dweller of the body while in the same body, so also the change of body befalls him; the wise one therefore does not become perplexed.
As said in 2:11 the wise do NOT grieve for the dead as for the living. Why?
Because Wise know that
1. Soul is eternal as said in 2:12.
2. One may still feel sad on account of loss of body. For this it is stated in 2:13 that wise know that just as living or embodied beings undergo change in bodies from childhood to old age, so do the dead change their bodies. The wise are aware of this and so do NOT grieve for the loss of body as well.