The river you stepped into yesterday is not the river you step into today. Heraclitus said this in Greece. The Buddha said the same thing in India. They were pointing at the same fire.
Anicca — impermanence — is one of the three marks of existence in Buddhist teaching. Everything that arises passes away. Every thought, every feeling, every body, every civilisation.
Our suffering comes largely from our attempts to freeze what is flowing. We want the pleasant experience to last. We want the painful experience to end. We want the people we love to stay exactly as they are.
Everything is on fire. See this clearly, and the fire begins to cool.
The contemplation of impermanence is not pessimism. It is radical honesty that opens the door to genuine appreciation. When you truly know that this moment will not come again, you begin to be present for it.
The Japanese have a word for this: mono no aware — the bittersweet pathos of things. The cherry blossoms are beautiful precisely because they fall.