Objection 1. It would seem that every passion decreases the goodness of a moral action. For anything that hinders the judgment of reason, on which depends the goodness of a moral act, consequently decreases the goodness of the moral act. But every passion hinders the judgment of reason: for Sallust says (Catilin.): "All those that take counsel about matters of doubt, should be free from hatred, anger, friendship and pity." Therefore passion decreases the goodness of a moral act.
Objection 2. Further, the more a man's action is like to God, the better it is: hence the Apostle says (Ephesians 5:1): "Be ye followers of God, as most dear children." But "God and the holy angels feel no anger when they punish... no fellow-feeling with misery when they relieve the unhappy," as Augustine says (De Civ. Dei ix, 5). Therefore it is better to do such like deeds without than with a passion of the soul.
Objection 3. Further, just as moral evil depends on its relation to reason, so also does moral good. But moral evil is lessened by passion: for he sins less, who sins from passion, than he who sins deliberately. Therefore he does a better deed, who does well without passion, than he who does with passion.
On the contrary, Augustine says (De Civ. Dei ix, 5) that "the passion of pity is obedient to reason, when pity is bestowed without violating right, as when the poor are relieved, or the penitent forgiven." But nothing that is obedient to reason lessens the moral good. Therefore a passion of the soul does not lessen moral good.
I answer that, As the Stoics held that every passion of the soul is evil, they consequently held that every passion of the soul lessens the goodness of an act; since the admixture of evil either destroys good altogether, or makes it to be less good. And this is true indeed, if by passions we understand none but the inordinate movements of the sensitive appetite, considered as disturbances or ailments. But if we give the name of passions to all the movements of the sensitive appetite, then it belongs to the perfection of man's good that his passions be moderated by reason. For since man's good is founded on reason as its root, that good will be all the more perfect, according as it extends to more things pertaining to man. Wherefore no one questions the fact that it belongs to the perfection of moral good, that the actions of the outward members be controlled by the law of reason. Hence, since the sensitive appetite can obey reason, as stated above (Question 17, Article 7), it belongs to the perfection of moral or human good, that the passions themselves also should be controlled by reason.
Accordingly just as it is better that man should both will good and do it in his external act; so also does it belong to the perfection of moral good, that man should be moved unto good, not only in respect of his will, but also in respect of his sensitive appetite; according to Psalm 83:3: "My heart and my flesh have rejoiced in the living God": where by "heart" we are to understand the intellectual appetite, and by "flesh" the sensitive appetite.
Reply to Objection 1. The passions of the soul may stand in a twofold relation to the judgment of reason. First, antecedently: and thus, since they obscure the judgment of reason, on which the goodness of the moral act depends, they diminish the goodness of the act; for it is more praiseworthy to do a work of charity from the judgment of reason than from the mere passion of pity. In the second place, consequently: and this in two ways. First, by way of redundance: because, to wit, when the higher part of the soul is intensely moved to anything, the lower part also follows that movement: and thus the passion that results in consequence, in the sensitive appetite, is a sign of the intensity of the will, and so indicates greater moral goodness. Secondly, by way of choice; when, to wit, a man, by the judgment of his reason, chooses to be affected by a passion in order to work more promptly with the co-operation of the sensitive appetite. And thus a passion of the soul increases the goodness of an action.
Reply to Objection 2. In God and the angels there is no sensitive appetite, nor again bodily members: and so in them good does not depend on the right ordering of passions or of bodily actions, as it does in us.
Reply to Objection 3. A passion that tends to evil, and precedes the judgment of reason, diminishes sin; but if it be consequent in either of the ways mentioned above (Reply to Objection 1), it aggravates the sin, or else it is a sign of its being more grievous.
Source: Summa Theologica (New Advent)