When we were young our concept of time was much different than it is now that we are older. Life seemed to go by so slow then and it really tested our patience. Today, life goes by so fast that there seems like no time left to get things done. When we were young we squandered our time pursuing frivolous things. Now that we are older we see the value in things.
When we realize we are not young anymore and that life is steadfastly going by, it teaches us to live our lives differently. Time, people and things become more valuable. There is an old proverb, "An inch of gold can't buy an inch of time." Once something is squandered it might not come back.
These days, I often wonder about the parable of the rice stalk that states, "When the young rice stalk sprouts, it stands up very straight; but as the rice stalk matures, it begins to bow its head towards the ground." Why does the mature rice stalk bow its head? Obviously, it bows because of humility. But what creates this sense of humility? I wonder if humility comes as a function of the realization of one's humanity. With this new found humanity sometimes comes the realization of the shortness of life and this in some ways is sad. This sense of sadness in Japanese is called kawaisou, but its not like a sadness in a depressive sense. Kawaisou is an inner reflection that brings a sense of sorrow upon the realization of something. In this case, it is that life is short and thus it causes one to hang their head.
When we realize that life is short we do our best to not waste time. In not wasting time, we come to see the value in life.