because he is scarce. The supply is limited and the demand is great; therefore, he is expensive.
You cannot bring him back. Once he passes by, he will never reverse his direction. As an ink, he is indelible. His steps permanent. His marks stamped forever.
Time is precious because the happiness of our future depends on the wise or unwise use of her. She gives back whatever he receives. He remembers without fail, he returns the favor, he never forgets, he could charge atrocities, and he could forgive.
Time is precious because he may stop at any moment. When he stops for some, his life ends. He is unpredictable.