Objection 1. It would seem that no passion of the soul is good or evil morally according to its species. Because moral good and evil depend on reason. But the passions are in the sensitive appetite; so that accordance with reason is accidental to them. Since, therefore, nothing accidental belongs to a thing's species, it seems that no passion is good or evil according to its species.
Objection 2. Further, acts and passions take their species from their object. If, therefore, any passion were good or evil, according to its species, it would follow that those passions the object of which is good, are specifically good, such as love, desire and joy: and that those passions, the object of which is evil, are specifically evil, as hatred, fear and sadness. But this is clearly false. Therefore no passion is good or evil according to its species.
Objection 3. Further, there is no species of passion that is not to be found in other animals. But moral good is in man alone. Therefore no passion of the soul is good or evil according to its species.
On the contrary, Augustine says (De Civ. Dei ix, 5) that "pity is a kind of virtue." Moreover, the Philosopher says (Ethic. ii, 7) that modesty is a praiseworthy passion. Therefore some passions are good or evil according to their species.
I answer that, We ought, seemingly, to apply to passions what has been said in regard to acts (18, 5,6; 20, 1)--viz. that the species of a passion, as the species of an act, can be considered from two points of view. First, according to its natural genus; and thus moral good and evil have no connection with the species of an act or passion. Secondly, according to its moral genus, inasmuch as it is voluntary and controlled by reason. In this way moral good and evil can belong to the species of a passion, in so far as the object to which a passion tends, is, of itself, in harmony or in discord with reason: as is clear in the case of "shame" which is base fear; and of "envy" which is sorrow for another's good: for thus passions belong to the same species as the external act.
Reply to Objection 1. This argument considers the passions in their natural species, in so far as the sensitive appetite is considered in itself. But in so far as the sensitive appetite obeys reason, good and evil of reason are no longer accidentally in the passions of the appetite, but essentially.
Reply to Objection 2. Passions having a tendency to good, are themselves good, if they tend to that which is truly good, and in like manner, if they turn away from that which is truly evil. On the other hand, those passions which consist in aversion from good, and a tendency to evil, are themselves evil.
Reply to Objection 3. In irrational animals the sensitive appetite does not obey reason. Nevertheless, in so far as they are led by a kind of estimative power, which is subject to a higher, i.e. the Divine reason, there is a certain likeness of moral good in them, in regard to the soul's passions.
Source: Summa Theologica (New Advent)