Because, as hope is about good things, so fear is about evil things. But experience conduces to the increase of hope in good things. Therefore it also adds to fear in evil things.
 Thomas Aquinas, Summa Theologica (1274). copy citation

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Author Thomas Aquinas
Source Summa Theologica
Topic evil fear
Date 1274
Language English
Reference
Note Translated by Fathers of the English Dominican Province
Weblink http://www.gutenberg.org/cache/epub/17897/pg17897-images.html

Context

“3: On account of the various kinds of fear already alluded to (ad 2) a man's present fear may have a future fear for its object. ________________________
FIFTH ARTICLE [I-II, Q. 42, Art. 5]
Whether Sudden Things Are Especially Feared?
Objection 1: It would seem that unwonted and sudden things are not especially feared. Because, as hope is about good things, so fear is about evil things. But experience conduces to the increase of hope in good things. Therefore it also adds to fear in evil things. Obj. 2: Further, the Philosopher says (Rhet. ii, 5) that "those are feared most, not who are quick-tempered, but who are gentle and cunning." Now it is clear that those who are quick-tempered are more subject to sudden emotions.” source