What does this quote mean?

“It is not that we have a short time to live, but that we waste a lot of it.”

Seneca shifts the complaint from life being too brief to attention and time being poorly spent.

Source note: English wording varies by translation.

“In a world where all is fleeting, change alone endures.”

Stability is temporary, so practical judgment requires expecting change rather than treating present arrangements as permanent.

Source note: Counsels and Maxims, T. Bailey Saunders translation, in a discussion of fortune and apparent stability.

“No one is so old as to think that he may not live a year.”

Even late in life people naturally continue to imagine a future, which helps explain why worthwhile work and planning do not simply stop with age.

Source note: Cato on Old Age, Shuckburgh translation, in a passage about older people planting for those who come after them.

“There is no cure for birth and death save to enjoy the interval.”

Mortality cannot be eliminated, so the practical answer is to make conscious use of the finite span between beginning and end.

Source note: Soliloquies in England, “War Shrines”; the sentence follows a contrast between strained worldliness and a more festive acceptance of life.

“To every thing there is a season, and a time to every purpose under the heaven.”

The verse places change inside a larger rhythm of seasons and purposes, making it useful for thinking about acceptance when one phase of life gives way to another.

Source note: Ecclesiastes 3:1 in the King James Version, opening the sequence of contrasting times and seasons.

“The course of life is fixed, and nature admits of its being run but in one way, and only once.”

Each stage of life occurs only once, so wisdom includes accepting its appropriate strengths instead of endlessly trying to restore an earlier stage.

Source note: Cato on Old Age, Shuckburgh translation, in Cicero’s discussion of the distinctive advantages of each season of life.