The problem is not that life is short but that we waste most of it; given properly, life is long enough to accomplish the greatest things. We do not receive a short life, we make it short by squandering it through neglect and distraction.
People absorbed in business, ambition, or pleasure live as if they will never die, postponing real living to a future that may not come. The result is that they are always preparing to live but never actually live, and old age arrives before they have begun.
People guard money and possessions jealously yet give away their time to anyone who asks, though time is the one thing that cannot be recovered once spent. They treat the most valuable and irreplaceable resource as if it were worthless.
Seneca cites the emperor Augustus, the most powerful man in the world, whose letters and speeches repeatedly express a weary longing for rest and freedom from public duties that he never obtained. Even absolute power could not buy him leisure, so he consoled himself with the mere hope of future ease.
The present is fleeting and the future uncertain, but the past is fixed and cannot be taken from us, so only those who use time well can recall their whole lives at will. The preoccupied, by contrast, dare not look back because their wasted past accuses them.