Objection 1. It would seem that every passion of the soul has a contrary. For every passion of the soul is either in the irascible or in the concupiscible faculty, as stated above (Article 1). But both kinds of passion have their respective modes of contrariety. Therefore every passion of the soul has its contrary.
Objection 2. Further, every passion of the soul has either good or evil for its object; for these are the common objects of the appetitive part. But a passion having good for its object, is contrary to a passion having evil for its object. Therefore every passion has a contrary.
Objection 3. Further, every passion of the soul is in respect of approach or withdrawal, as stated above (Article 2). But every approach has a corresponding contrary withdrawal, and vice versa. Therefore every passion of the soul has a contrary.
On the contrary, Anger is a passion of the soul. But no passion is set down as being contrary to anger, as stated in Ethic. iv, 5. Therefore not every passion has a contrary.
I answer that, The passion of anger is peculiar in this, that it cannot have a contrary, either according to approach and withdrawal, or according to the contrariety of good and evil. For anger is caused by a difficult evil already present: and when such an evil is present, the appetite must needs either succumb, so that it does not go beyond the limits of "sadness," which is a concupiscible passion; or else it has a movement of attack on the hurtful evil, which movement is that of "anger." But it cannot have a movement of withdrawal: because the evil is supposed to be already present or past. Thus no passion is contrary to anger according to contrariety of approach and withdrawal.
In like manner neither can there be according to contrariety of good and evil. Because the opposite of present evil is good obtained, which can be no longer have the aspect of arduousness or difficulty. Nor, when once good is obtained, does there remain any other movement, except the appetite's repose in the good obtained; which repose belongs to joy, which is a passion of the concupiscible faculty.
Accordingly no movement of the soul can be contrary to the movement of anger, and nothing else than cessation from its movement is contrary thereto; thus the Philosopher says (Rhet. ii, 3) that "calm is contrary to anger," by opposition not of contrariety but of negation or privation.
From what has been said the replies to the objections are evident.
Source: Summa Theologica (New Advent)