The psychological causes of unhappiness, it is clear, are many and various. But all have something in common. The typical unhappy man is one who, having been deprived in youth of some normal satisfaction, has come to value this one kind of satisfaction more than any other, and has therefore given to his life a one-sided direction, together with a quite undue emphasis upon the achievement as opposed to the activities connected with it.
Source: The Conquest of Happiness, 1930, chap.１：What makes people unhappy?
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