Those who make the worst use of their time are the first to complain of its shortness.
I've been quite fascinated by the relative insignificance of human existence the shortness of life. We might as well be a letter in a word in a sentence on a page in a book in a library in a city in one country in this enormous universe! And that kind of fear and insignificance has kept me awake at night.
Without fullness of experience length of days is nothing. When fullness of life has been achieved shortness of days is nothing. That is perhaps why the young have usually so little fear of death they live by intensities that the elderly have forgotten.