The tides are a gentle ebb and flow in nature, the phrase ebb and flow is used to described similar circumstances in life. For example, over long periods, relationships and marriages ebb and flow from periods of intense happiness to lows of sorrow. If these changes in circumstances occur more rapidly, they would be referred to as a “roller coaster ride” because the ups and downs would be more frequent and violent. Ebb and flow is intended to describe more gentle, natural changes in things that have high points and low points.

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RE:
What does the term "ebb and flow" mean? Where does it come from? How is it used?

A decline and increase, constant fluctuations. For example, He was fascinated by the ebb and flow of the Church’s influence over the centuries. This expression alludes to the inward and outward movement of ocean tides. [Late 1500s]

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It comes from Ocean Waves, Tides and Current.