Every spiritual tradition has stressed that this human life is unique, and has potential that ordinarily we hardly even begin to imagine. If we miss the opportunity this life offers us for transforming ourselves, they say, it may well be an extremely long time before we have another. Imagine a blind turtle, roaming the depths of an ocean the size of the universe. Up above floats a wooden ring, tossed to and fro on the waves. Every hundred years the turtle comes, once, to the surface. To be born a human being is said by Buddhists to be more difficult than for the turtle to surface accidentally with its head poking through the wooden ring. And even among those who have a human birth, it is said, those who have the great fortune to make a connection with the teachings are rare; and those who really take them to heart and embody them in their actions even rarer, as rare, in fact, "as stars in broad daylight."
Sogyal Rinpoche, writer & Buddhist teacher, The Tibetan Book of Living and Dying