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.
Objection 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. Therefore sudden things are less to be feared.
Objection 3. Further, we think less about things that happen suddenly. But the more we think about a thing, the more we fear it; hence the Philosopher says (Ethic. iii, 8) that "some appear to be courageous through ignorance, but as soon as they discover that the case is different from what they expected, they run away." Therefore sudden things are feared less.
On the contrary, Augustine says (Confess. ii, 6): "Fear is startled at things unwonted and sudden, which endanger things beloved, and takes forethought for their safety."
I answer that, As stated about (3; 41, 2), the object of fear is an imminent evil, which can be repelled, but with difficulty. Now this is due to one of two causes: to the greatness of the evil, or to the weakness of him that fears; while unwontedness and suddenness conduce to both of these causes. First, it helps an imminent evil to seem greater. Because all material things, whether good or evil, the more we consider them, the smaller they seem. Consequently, just as sorrow for a present evil is mitigated in course of time, as Cicero states (De Quaest. Tusc. iii, 30); so, too, fear of a future evil is diminished by thinking about it beforehand. Secondly, unwontedness and suddenness increase the weakness of him that fears, in so far as they deprive him of the remedies with which he might otherwise provide himself to forestall the coming evil, were it not for the evil taking him by surprise.
Reply to Objection 1. The object of hope is a good that is possible to obtain. Consequently whatever increases a man's power, is of a nature to increase hope, and, for the same reason, to diminish fear, since fear is about an evil which cannot be easily repelled. Since, therefore, experience increases a man's power of action, therefore, as it increases hope, so does it diminish fear.
Reply to Objection 2. Those who are quick-tempered do not hide their anger; wherefore the harm they do others is not so sudden, as not to be foreseen. On the other hand, those who are gentle or cunning hide their anger; wherefore the harm which may be impending from them, cannot be foreseen, but takes one by surprise. For this reason the Philosopher says that such men are feared more than others.
Reply to Objection 3. Bodily good or evil, considered in itself, seems greater at first. The reason for this is that a thing is more obvious when seen in juxtaposition with its contrary. Hence, when a man passes unexpectedly from penury to wealth, he thinks more of his wealth on account of his previous poverty: while, on the other hand, the rich man who suddenly becomes poor, finds poverty all the more disagreeable. For this reason sudden evil is feared more, because it seems more to be evil. However, it may happen through some accident that the greatness of some evil is hidden; for instance if the foe hides himself in ambush: and then it is true that evil inspires greater fear through being much thought about.
Source: Summa Theologica (New Advent)