"This was explained by the Buddha in the sutras:

"Suppose that this whole earth were an ocean and a person threw in a yoke that had only one hole. The yoke would float back and forth in all the four directions. Underneath that ocean, there is a blind tortoise who lives for many thousands of years but who comes up above the surface once every hundred years. It would be very difficult for the tortoise's head to meet with the yoke's hole; still, it is possible. To be born in a precious human life is much more difficult."

Why is a precious human life difficult to find? This body of leisure and endowments is gained through the accumulation of virtuous deeds, and those who are born in the three lower realms do not know how to accumulate virtue. Rather, they constantly commit evil deeds. Therefore, only those born in the three lower realms with a very small amount of negative karma, and whose karma could ripen in another lifetime, are the ones who have an opportunity to be born in a human life.

In Sanskrit, "man" is purusha, which translates as "capacity" or "ability." Hence, a human life with the qualities of leisure and endowment provides the capacity or ability to attain either the temporary high realms or definite goodness; therefore, it is called purusha."

Gampopa