Objection 1. It would seem that some defect is a cause of daring. For the Philosopher says (De Problem. xxvii, 4) that "lovers of wine are strong and daring." But from wine ensues the effect of drunkenness. Therefore daring is caused by a defect.
Objection 2. Further, the Philosopher says (Rhet. ii, 5) that "those who have no experience of danger are bold." But want of experience is a defect. Therefore daring is caused by a defect.
Objection 3. Further, those who have suffered wrongs are wont to be daring; "like the beasts when beaten," as stated in Ethic. iii, 5. But the suffering of wrongs pertains to defect. Therefore daring is caused by a defect.
On the contrary, The Philosopher says (Rhet. ii, 5) that the cause of daring "is the presence in the imagination of the hope that the means of safety are nigh, and that the things to be feared are either non-existent or far off." But anything pertaining to defect implies either the removal of the means of safety, or the proximity of something to be feared. Therefore nothing pertaining to defect is a cause of daring.
I answer that, As stated above (1,2) daring results from hope and is contrary to fear: wherefore whatever is naturally apt to cause hope or banish fear, is a cause of daring. Since, however, fear and hope, and also daring, being passions, consist in a movement of the appetite, and in a certain bodily transmutation; a thing may be considered as the cause of daring in two ways, whether by raising hope, or by banishing fear; in one way, in the part of the appetitive movement; in another way, on the part of the bodily transmutation.
On the part of the appetitive movement which follows apprehension, hope that leads to daring is roused by those things that make us reckon victory as possible. Such things regard either our own power, as bodily strength, experience of dangers, abundance of wealth, and the like; or they regard the powers of others, such as having a great number of friends or any other means of help, especially if a man trust in the Divine assistance: wherefore "those are more daring, with whom it is well in regard to godlike things," as the Philosopher says (Rhet. ii, 5). Fear is banished, in this way, by the removal of threatening causes of fear; for instance, by the fact that a man has not enemies, through having harmed nobody, so that he is not aware of any imminent danger; since those especially appear to be threatened by danger, who have harmed others.
On the part of the bodily transmutation, daring is caused through the incitement of hope and the banishment of fear, by those things which raise the temperature about the heart. Wherefore the Philosopher says (De Part. Animal. iii, 4) that "those whose heart is small in size, are more daring; while animals whose heart is large are timid; because the natural heat is unable to give the same degree of temperature to a large as to a small heart; just as a fire does not heat a large house as well as it does a small house." He says also (De Problem. xxvii, 4), that "those whose lungs contain much blood, are more daring, through the heat in the heart that results therefrom." He says also in the same passage that "lovers of wine are more daring, on account of the heat of the wine": hence it has been said above (Question 40, Article 6) that drunkenness conduces to hope, since the heat in the heart banishes fear and raises hope, by reason of the dilatation and enlargement of the heart.
Reply to Objection 1. Drunkenness causes daring, not through being a defect, but through dilating the heart: and again through making a man think greatly of himself.
Reply to Objection 2. Those who have no experience of dangers are more daring, not on account of a defect, but accidentally, i.e. in so far as through being inexperienced they do not know their own failings, nor the dangers that threaten. Hence it is that the removal of the cause of fear gives rise to daring.
Reply to Objection 3. As the Philosopher says (Rhet. ii, 5) "those who have been wronged are courageous, because they think that God comes to the assistance of those who suffer unjustly."
Hence it is evident that no defect causes daring except accidentally, i.e. in so far as some excellence attaches thereto, real or imaginary, either in oneself or in another.
Source: Summa Theologica (New Advent)