Seneca was particularly skeptical of the double-edged sword of achievement and ambition, which cause us to steep in our cesspool of insecurity, dissatisfaction, and clinging.
It is inevitable that life will be not only very short but very miserable for those who acquire by great toil what they must keep by greater toil. They achieve what they want laboriously; they possess what they have achieved anxiously; and meanwhile they take no account of time that will never more return. New preoccupations take the place of the old, hope excites more hope and ambition more ambition. They do not look for an end to their misery, but simply change the reason for it.
This, Seneca cautions, is tenfold more toxic for the soul when one is working for someone else, toiling away toward goals laid out by another.
When you are young it is natural to be ambitious and to try to achieve great things. But if you remain that way at 70 or 80 you have learned little from life and are probably difficult to live with. For most of us this striving seems rather pointless.