There are some people who don't enjoy the available good time and sometimes it even worries them. I am not sure why, but they might think that they will miss those moment and suffer for the lack of it.
Learn English – the idiom or expression to describe the state that a person interrupts their happy time by believing that “this will end soon “
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Best Answer
One U.S. idiom for a person who constantly frets even when nothing is seriously wrong is worrywart. Here is the entry for that word in Barbara Kipfer & Robert Chapman, Dictionary of American Slang, fourth edition (2007):
Rosalind Fergusson, The Facts on File Dictionary of Proverbs (1983) has a relevant saying about people who can't relax and enjoy a pleasant situation:
The bracketed comments in the two quotations above appear in the original texts.
Charles Doyle, Wolfgang Mieder, and Fred Shapiro, The Dictionary of Modern Proverbs (2012) has two sayings that address the senselessness of endless worrying:
(which the dictionary traces to a collection of sayings from 1909) and
(which it dates to 1916).
The state in which a worrywart is most comfortable (or uncomfortable) is sometimes termed worrywartism. From David Sue & Stanley Sue, Abnormal Behavior (1990) [combined snippets]:
So to describe the state of a person who can't fully enjoy a happy time because "this too shall pass," you could use the (somewhat unusual) idiomatic term "worrywartism" or the (somewhat specialized) clinical term "generalized anxiety disorder."