Objection 1. It would seem that those things are not more to be feared, for which there is no remedy. Because it is a condition of fear, that there be some hope of safety, as stated above (Article 2). But an evil that cannot be remedied leaves no hope of escape. Therefore such things are not feared at all.
Objection 2. Further, there is no remedy for the evil of death: since, in the natural course of things, there is no return from death to life. And yet death is not the most feared of all things, as the Philosopher says (Rhet. ii, 5). Therefore those things are not feared most, for which there is no remedy.
Objection 3. Further, the Philosopher says (Ethic. i, 6) that "a thing which lasts long is no better than that which lasts but one day: nor is that which lasts for ever any better than that which is not everlasting": and the same applies to evil. But things that cannot be remedied seem to differ from other things, merely in the point of their lasting long or for ever. Consequently they are not therefore any worse or more to be feared.
On the contrary, the Philosopher says (Rhet. ii, 5) that "those things are most to be feared which when done wrong cannot be put right... or for which there is no help, or which are not easy."
I answer that, The object of fear is evil: consequently whatever tends to increase evil, conduces to the increase of fear. Now evil is increased not only in its species of evil, but also in respect of circumstances, as stated above (Question 18, Article 3). And of all the circumstances, longlastingness, or even everlastingness, seems to have the greatest bearing on the increase of evil. Because things that exist in time are measured, in a way, according to the duration of time: wherefore if it be an evil to suffer something for a certain length of time, we should reckon the evil doubled, if it be suffered for twice that length of time. And accordingly, to suffer the same thing for an infinite length of time, i.e. for ever, implies, so to speak, an infinite increase. Now those evils which, after they have come, cannot be remedied at all, or at least not easily, are considered as lasting for ever or for a long time: for which reason they inspire the greatest fear.
Reply to Objection 1. Remedy for an evil is twofold. One, by which a future evil is warded off from coming. If such a remedy be removed, there is an end to hope and consequently to fear; wherefore we do not speak now of remedies of that kind. The other remedy is one by which an already present evil is removed: and of such a remedy we speak now.
Reply to Objection 2. Although death be an evil without remedy, yet, since it threatens not from near, it is not feared, as stated above (Article 2).
Reply to Objection 3. The Philosopher is speaking there of things that are good in themselves, i.e. good specifically. And such like good is no better for lasting long or for ever: its goodness depends on its very nature.
Source: Summa Theologica (New Advent)