As long as a man is in the full vigor of life, he forgets the naked truth of death, which he has to meet. Thus a foolish man makes no relevant inquiry about the real problems of life. Everyone thinks that he will never die, although he sees evidence of death before his eyes at every second. Here is the distinction between animalism and humanity.
An animal like a goat has no sense of its impending death. Although its brother goat is being slaughtered, the goat, being allured by the green grass offered to it, will stand peacefully waiting to be slaughtered next. On the other hand, if a human being sees his fellow man being killed by an enemy, he either fights to save his brother or leaves, if possible, to save his own life. That is the difference between a man and a goat.
An intelligent man knows that death is born along with his own birth. He knows that he is dying at every second and that the final touch will be given as soon as his term of life is finished. He therefore prepares himself for the next life or for liberation from the disease of repeated birth and death.
A foolish man, however, does not know that this human form of life is obtained after a series of births and deaths imposed in the past by the laws of nature. He does not know that a living entity is an eternal being, who has no birth and death. Birth, death, old age, and disease are external impositions on a living entity and are due to his contact with material nature and to his forgetfulness of his eternal, godly nature and qualitative oneness with the Absolute Whole.
Human life provides the opportunity to know this eternal fact, or truth. Thus the very beginning of the Vedanta-sutra advises that because we have this valuable form of human life, it is our duty-now-to inquire, What is Brahman, the Absolute Truth?
Ref: Chapter 1f Science of Self Realization authored by Srila Prabhupada (Founder – Acharya, ISKCON)