Objection 1. It would seem that all the passions of the soul are morally evil. For Augustine says (De Civ. Dei ix, 4) that "some call the soul's passions diseases or disturbances of the soul" [Those things which the Greeks call pathe, we prefer to call disturbances rather than diseases (Tusc. iv. 5)]. But every disease or disturbance of the soul is morally evil. Therefore every passion of the soul is evil morally.
Objection 2. Further, Damascene says (De Fide Orth. ii, 22) that "movement in accord with nature is an action, but movement contrary to nature is passion." But in movements of the soul, what is against nature is sinful and morally evil: hence he says elsewhere (De Fide Orth. ii, 4) that "the devil turned from that which is in accord with nature to that which is against nature." Therefore these passions are morally evil.
Objection 3. Further, whatever leads to sin, has an aspect of evil. But these passions lead to sin: wherefore they are called "the passions of sins" (Romans 7:5). Therefore it seems that they are morally evil.
On the contrary, Augustine says (De Civ. Dei xiv, 9) that "all these emotions are right in those whose love is rightly placed... For they fear to sin, they desire to persevere; they grieve for sin, they rejoice in good works."
I answer that, On this question the opinion of the Stoics differed from that of the Peripatetics: for the Stoics held that all passions are evil, while the Peripatetics maintained that moderate passions are good. This difference, although it appears great in words, is nevertheless, in reality, none at all, or but little, if we consider the intent of either school. For the Stoics did not discern between sense and intellect; and consequently neither between the intellectual and sensitive appetite. Hence they did not discriminate the passions of the soul from the movements of the will, in so far as the passions of the soul are in the sensitive appetite, while the simple movements of the will are in the intellectual appetite: but every rational movement of the appetitive part they call will, while they called passion, a movement that exceeds the limits of reason. Wherefore Cicero, following their opinion (De Tusc. Quaest. iii, 4) calls all passions "diseases of the soul": whence he argues that "those who are diseased are unsound; and those who are unsound are wanting in sense." Hence we speak of those who are wanting in sense of being "unsound."
On the other hand, the Peripatetics give the name of "passions" to all the movements of the sensitive appetite. Wherefore they esteem them good, when they are controlled by reason; and evil when they are not controlled by reason. Hence it is evident that Cicero was wrong in disapproving (De Tusc. Quaest. iii, 4) of the Peripatetic theory of a mean in the passions, when he says that "every evil, though moderate, should be shunned; for, just as a body, though it be moderately ailing, is not sound; so, this mean in the diseases or passions of the soul, is not sound." For passions are not called "diseases" or "disturbances" of the soul, save when they are not controlled by reason.
Hence the reply to the First Objection is evident.
Reply to Objection 2. In every passion there is an increase or decrease in the natural movement of the heart, according as the heart is moved more or less intensely by contraction and dilatation; and hence it derives the character of passion. But there is no need for passion to deviate always from the order of natural reason.
Reply to Objection 3. The passions of the soul, in so far as they are contrary to the order of reason, incline us to sin: but in so far as they are controlled by reason, they pertain to virtue.
Source: Summa Theologica (New Advent)