Kant says it is man's rational duty
Mill and Betham, because of pleasure and happiness
Fletcher, it depends upon the situation
St. Augustine and Aquinas, because there is God
Aristotle, Plato, and Socrates, because of happiness
Hindu seers, because of Karma to be one with the Brahman
Lao-Tzu and Confucius, because of man's deepest pleading to be in harmony with nature
For Muhammad, because of Allah
Nietzsche, because God had nothing to do with man's quest