Quotation by Bertrand Russell:

Pleasures of achievement demand difficulties such that beforehand success seems doubtful although in the end it is usually achieved. This is perhaps the chief reason why a modest estimate of one's own powers is a source of happiness. The man who underestimates himself is perpetually being surprised by success, whereas the man who overestimates himself is just as often surprised by failure. The former kind of surprise is pleasant, the latter unpleasant.

Bertrand Russell     (more by this author)

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Source: The Conquest of Happiness