IT WILL PASS

You can never set foot in the same river twice.  It is not the same river the second time.  Existence is changing, ever changing.  Since all is transient this way, all is unreliable. This is called "annica" in Buddhism, the First Dharma Seal. When a prince asked his jeweler to create something for him that would carry him through the good times as well as the bad, the jeweler made him a ring inscribed with the words "It will pass."

This impermanence results in frustration, hence suffering.  Life is a moving target, which is moving every which way and at every possible speed.  Meanwhile the well-intentioned shooter, the individual in the world, is also moving every which way and at every possible speed, but not always in the same direction or at same speed as the target.  Everyone and everything is in this state of hit-and-miss. 

Buddhism is not the only place where this observation is found.  In western philosophy, Heraclitus (c.a. 535-475 B.C.) spoke of "flux."  Nothing possesses the permanency of "being," he said.  All is in a state of “becoming,” of becoming something else.  Nothing is permanent except change, he said.  In the same way, the person reading this posting is not the same person who will finish it.  Thomas Merton, the 20th century American Catholic writer, when speaking of his early autobiography THE SEVEN STOREY MOUNTAIN, said that it was somebody else who wrote it, a different Thomas Merton.  "Mindfulness" in Buddhism is being aware this transience.