Yam yam vaapi smaran bhaavam tyajatyante kalevaram
tam tamevaiti kaunteya sadaa tadbhaavabhaavitah
(Whosoever at the end leaves the body, thinking of any being, to that being only does he go, O son of Kunti, because of his constant thought of that being!)
— Bhagavad Gita, Chapter 8, Verse 6
Lord Krishna says to Arjun that whatever is the thought of the being when he leaves the body at the time of death, that and that alone one attains, being absorbed in its thought forever. The body here does not refer to what you see in the mirror but extends beyond to include the five sheaths — annamaya, pranamaya, manomaya, vijnanamaya and anandamaya. An ordinary being desires only the physical and exists at the level of the annamaya (physical body), which in turn is controlled by the pranamaya kosha comprising of prana. The soul stays in the body till the time there is prana. Death is a state when there is no prana left in the body and hence the soul leaves the body.
The soul might leave the body, but it is still tied to its desires and karmas. There’s a story that I had heard about the 1943 famine in Bengal. It saw three million die for the want of food. In those times, people had reported mysterious rattling of vessels in the kitchen. What made the vessels rattle? It was the hungry spirits of those who passed away, their desire for food was so strong that as soon as they left the body, they took shelter in the vessels where food was kept. Alexander the Great had asked his general to leave his empty hands hang out of the coffin upon his death to show the world that you enter and exit empty-handed. But Vedic sages knew that you take with you your desires and karmas and they decide your next birth and body.
So if all your life you have thought of just yourself and done nothing for anyone else, that is, then that will be your thought at the time of exiting the body as well, and the next body will be in as per that, maybe the body of an animal, for that is the nature of an animal. The nature of humans is to protect and provide for those weaker than them, and the one who fulfils this is entitled to higher births and subtler bodies, says the Gita.
