Born in 121 AD, Marcus Aurelius was a Stoic philosopher in ancient Rome and was, at one time, the most powerful man in the world. In many ways he was history's most quintessential example of Plato's ideal of the "philosopher king"—a Roman emperor who did not let power corrupt him and strove day in and day out to live a wise and virtuous life.
This succinct statement from Marcus is a potent, striking reminder of flux, impermanence. Everything is constantly changing; all things will fall away in time. Thus, to expect things to stay the same is to set ourselves up for immense suffering. By accepting impermanence, we render ourselves much more capable of flowing with the tides of fate, and we become much more appreciative for our present blessings and loved ones, understanding that they will eventually fade away. Thus, contemplating impermanence—or imagining the loss of things we hold dear, as the Stoics did—is a profoundly powerful practice for reducing our own suffering and increasing our joy and appreciation for the present.