Objection 1. It would seem that a man's excellence is not the cause of his being more easily angry. For the Philosopher says (Rhet. ii, 2) that "some are angry especially when they are grieved, for instance, the sick, the poor, and those who are disappointed." But these things seem to pertain to defect. Therefore defect rather than excellence makes one prone to anger.
Objection 2. Further, the Philosopher says (Rhet. ii, 2) that "some are very much inclined to be angry when they are despised for some failing or weakness of the existence of which there are grounds for suspicion; but if they think they excel in those points, they do not trouble." But a suspicion of this kind is due to some defect. Therefore defect rather than excellence is a cause of a man being angry.
Objection 3. Further, whatever savors of excellence makes a man agreeable and hopeful. But the Philosopher says (Rhet. ii, 3) that "men are not angry when they play, make jokes, or take part in a feast, nor when they are prosperous or successful, nor in moderate pleasures and well-founded hope." Therefore excellence is not a cause of anger.
On the contrary, The Philosopher says (Rhet. ii, 9) that excellence makes men prone to anger.
I answer that, The cause of anger, in the man who is angry, may be taken in two ways. First in respect of the motive of anger: and thus excellence is the cause of a man being easily angered. Because the motive of anger is an unjust slight, as stated above (Article 2). Now it is evident that the more excellent a man is, the more unjust is a slight offered him in the matter in which he excels. Consequently those who excel in any matter, are most of all angry, if they be slighted in that matter; for instance, a wealthy man in his riches, or an orator in his eloquence, and so forth.
Secondly, the cause of anger, in the man who is angry, may be considered on the part of the disposition produced in him by the motive aforesaid. Now it is evident that nothing moves a man to anger except a hurt that grieves him: while whatever savors of defect is above all a cause of grief; since men who suffer from some defect are more easily hurt. And this is why men who are weak, or subject to some other defect, are more easily angered, since they are more easily grieved.
This suffices for the Reply to the First Objection.
Reply to Objection 2. If a man be despised in a matter in which he evidently excels greatly, he does not consider himself the loser thereby, and therefore is not grieved: and in this respect he is less angered. But in another respect, in so far as he is more undeservedly despised, he has more reason for being angry: unless perhaps he thinks that he is envied or insulted not through contempt but through ignorance, or some other like cause.
Reply to Objection 3. All these things hinder anger in so far as they hinder sorrow. But in another respect they are naturally apt to provoke anger, because they make it more unseemly to insult anyone.
Source: Summa Theologica (New Advent)