Objection 1. It seems that the sin of fear is not contrary to fortitude: because fortitude is about dangers of death, as stated above (123, A4,5). But the sin of fear is not always connected with dangers of death, for a gloss on Psalm 127:1, "Blessed are all they that fear the Lord," says that "it is human fear whereby we dread to suffer carnal dangers, or to lose worldly goods." Again a gloss on Matthew 27:44, "He prayed the third time, saying the selfsame word," says that "evil fear is threefold, fear of death, fear of pain, and fear of contempt." Therefore the sin of fear is not contrary to fortitude.
Objection 2. Further, the chief reason why a man is commended for fortitude is that he exposes himself to the danger of death. Now sometimes a man exposes himself to death through fear of slavery or shame. Thus Augustine relates (De Civ. Dei i) that Cato, in order not to be Caesar's slave, gave himself up to death. Therefore the sin of fear bears a certain likeness to fortitude instead of being opposed thereto.
Objection 3. Further, all despair arises from fear. But despair is opposed not to fortitude but to hope, as stated above (20, 1; I-II, 40, 4). Neither therefore is the sin of fear opposed to fortitude.
On the contrary, The Philosopher (Ethic. ii, 7; iii, 7) states that timidity is opposed to fortitude.
I answer that, As stated above (19, 3; I-II, 43, 1), all fear arises from love; since no one fears save what is contrary to something he loves. Now love is not confined to any particular kind of virtue or vice: but ordinate love is included in every virtue, since every virtuous man loves the good proper to his virtue; while inordinate love is included in every sin, because inordinate love gives use to inordinate desire. Hence in like manner inordinate fear is included in every sin; thus the covetous man fears the loss of money, the intemperate man the loss of pleasure, and so on. But the greatest fear of all is that which has the danger of death for its object, as we find proved in Ethic. iii, 6. Wherefore the inordinateness of this fear is opposed to fortitude which regards dangers of death. For this reason timidity is said to be antonomastically* opposed to fortitude. [Antonomasia is the figure of speech whereby we substitute the general for the individual term; e.g. The Philosopher for Aristotle: and so timidity, which is inordinate fear of any evil, is employed to denote inordinate fear of the danger of death.]
Reply to Objection 1. The passages quoted refer to inordinate fear in its generic acceptation, which can be opposed to various virtues.
Reply to Objection 2. Human acts are estimated chiefly with reference to the end, as stated above (I-II, 01, 3; I-II, 18, 6): and it belongs to a brave man to expose himself to danger of death for the sake of a good. But a man who exposes himself to danger of death in order to escape from slavery or hardships is overcome by fear, which is contrary to fortitude. Hence the Philosopher says (Ethic. iii, 7), that "to die in order to escape poverty, lust, or something disagreeable is an act not of fortitude but of cowardice: for to shun hardships is a mark of effeminacy."
Reply to Objection 3. As stated above (I-II, 45, 2), fear is the beginning of despair even as hope is the beginning of daring. Wherefore, just as fortitude which employs daring in moderation presupposes hope, so on the other hand despair proceeds from some kind of fear. It does not follow, however, that any kind of despair results from any kind of fear, but that only from fear of the same kind. Now the despair that is opposed to hope is referred to another kind, namely to Divine things; whereas the fear that is opposed to fortitude regards dangers of death. Hence the argument does not prove.
Source: Summa Theologica (New Advent)