The second renunciation thought is impermanence. When we look closely at our environment and the beings within it we see that everything and everyone is changing. Nothing stays the same—including ourselves. This is the way nature is, the way of relative truth.

The teachings say that among all conceptions, the supreme conception is impermanence. In other words, thinking of impermanence is the best, most useful thought, because it is the most truthful.

As an example of impermanence, we can look at the movement of time. January merges into February, February becomes March, and like this the months keep passing through December until the year ends. January returns, and with it another year begins that will also end. On and on like this it goes. Time moves swiftly.

This is also true for individual beings. It has been said many times that the span of a single life is like a book with a finite number of pages. Each page we turn brings us closer to the end of a chapter, and a new chapter begins.

Right now we are in this chapter. We should read every word with care so that when we finish it our hearts and minds are full of wisdom. Then we can begin the next chapter bathed in brilliant, beautiful sunshine. That is our plan, and that is the purpose of the teachings and the practices.

Time is wonderful and precious and limited, and it is in your hands. Use it well, as all the great masters did before you. With courage and commitment, continue your practice. Do not waste this opportunity.

Of course, we have many things to do in the relative world, but along with these, we should develop our inner strength and realization. We should develop internally at least as much as we do on the external level, and perhaps the internal level should be emphasized even more.

Therefore, please reflect on impermanence. It will strengthen your joyful effort and motivate you to strive for and attain what is positive and everlasting. When you appreciate the power of your time and acknowledge its fleeting nature, you will not waste it; you will use it well.

Then, when your time ends—when the book of this life is finished, and your body and mind separate—you will not be disappointed.

Source: Sherab, Khenchen. The Nature of Mind (p. 120). Shambhala. Kindle Edition.

If this support [precious human life], with its freedoms and advantages, so extremely difficult to find, were something that, once obtained, never died, that would be fine.
But it is not eternal—far from it—and you are certain to die. Everyone thinks, “I will die eventually,” but up until the moment of death they go on hoping they will not die for a while.
Obscured by this sort of wrong thinking, they pass the time being distracted by the things of this life. They put off accomplishing anything for the next life until later and are overpowered by laziness.
This is why we have to meditate on impermanence.
As the Discourse on Impermanence says,
In front of everyone who is born,
Waiting there, is death,
And even I am not exempt.
Practice Dharma, therefore, from this very day.
(Dudjom Rinpoche)