The Bodhisatta was once Indra, king of the gods. An ascetic who lived in the Himalayas adopted an orphaned fawn and treated it as a child. One day the deer ate too much grass and died of indigestion. The ascetic became depressed, and when the Bodhisatta noticed this he came down to Earth and floated in the air, telling him that his sorrow was improper. This reminded the ascetic about the impermanence of all things, which restored his happiness.
In the Lifetime of the Buddha
The ascetic and the deer were earlier births of one of the Buddha’s elder disciples and a novice under his guidance. The elder was overcome with grief after the novice got sick and died. When the Buddha heard some other disciples discussing the elder’s sorrow, he told them this story so they knew the elder had also lost control of his emotions and lamented the death of a youngster in his care in the past.