What does it mean when we say, “What goes up must come down”?
On the surface, this common English idiom refers to the law of gravity, on Earth. If you throw something upward, it must come down, due to the Earth’s gravity.
On a deeper level, it refers to the universal principle that everything goes through cycles—stocks, weather, emotions, etc. It implies that anything which goes far in one direction, must eventually reverse; if you feel energetic, you must become tired. It is similar to the Chinese Taoist idea of yin and yang.
However, the phrase has further, more Western, implications. It begins with going up, which we assume is desirable; we want more money, more health, more power, etc. And then, it gives the warning, must come down, the word down having largely negative connotations.
It’s a phrase you might hear used as a warning for bitcoin investors! In this respect, it also echoes the Hindu idea of karma; the consequence of going up is that you must come down. It is similar to the saying “the taller they are, the harder they fall.”
As far as word history, this phrase first appeared in print in the early 19th century and it remains a commonly used phrase.
So to summarize, “what comes up must come down” points to the cyclic nature of life, with an emphasized warning, using the idea of gravity as a metaphor.