Objection 1. It would seem that envy is not a kind of sorrow. For the object of envy is a good, for Gregory says (Moral. v, 46) of the envious man that "self-inflicted pain wounds the pining spirit, which is racked by the prosperity of another." Therefore envy is not a kind of sorrow.
Objection 2. Further, likeness is a cause, not of sorrow but rather of pleasure. But likeness is a cause of envy: for the Philosopher says (Rhet. ii, 10): "Men are envious of such as are like them in genus, in knowledge, in stature, in habit, or in reputation." Therefore envy is not a kind of sorrow.
Objection 3. Further, sorrow is caused by a defect, wherefore those who are in great defect are inclined to sorrow, as stated above (I-II, 47, 3) when we were treating of the passions. Now those who lack little, and who love honors, and who are considered wise, are envious, according to the Philosopher (Rhet. ii, 10). Therefore envy is not a kind of sorrow.
Objection 4. Further, sorrow is opposed to pleasure. Now opposite effects have not one and the same cause. Therefore, since the recollection of goods once possessed is a cause of pleasure, as stated above (I-II, 32, 3) it will not be a cause of sorrow. But it is a cause of envy; for the Philosopher says (Rhet. ii, 10) that "we envy those who have or have had things that befitted ourselves, or which we possessed at some time." Therefore sloth is not a kind of sorrow.
On the contrary, Damascene (De Fide Orth. ii, 14) calls envy a species of sorrow, and says that "envy is sorrow for another's good."
I answer that, The object of a man's sorrow is his own evil. Now it may happen that another's good is apprehended as one's own evil, and in this way sorrow can be about another's good. But this happens in two ways: first, when a man is sorry about another's good, in so far as it threatens to be an occasion of harm to himself, as when a man grieves for his enemy's prosperity, for fear lest he may do him some harm: such like sorrow is not envy, but rather an effect of fear, as the Philosopher states (Rhet. ii, 9).
Secondly, another's good may be reckoned as being one's own evil, in so far as it conduces to the lessening of one's own good name or excellence. It is in this way that envy grieves for another's good: and consequently men are envious of those goods in which a good name consists, and about which men like to be honored and esteemed, as the Philosopher remarks (Rhet. ii, 10).
Reply to Objection 1. Nothing hinders what is good for one from being reckoned as evil for another: and in this way it is possible for sorrow to be about good, as stated above.
Reply to Objection 2. Since envy is about another's good name in so far as it diminishes the good name a man desires to have, it follows that a man is envious of those only whom he wishes to rival or surpass in reputation.
But this does not apply to people who are far removed from one another: for no man, unless he be out of his mind, endeavors to rival or surpass in reputation those who are far above him. Thus a commoner does not envy the king, nor does the king envy a commoner whom he is far above. Wherefore a man envies not those who are far removed from him, whether in place, time, or station, but those who are near him, and whom he strives to rival or surpass. For it is against our will that these should be in better repute than we are, and that gives rise to sorrow. On the other hand, likeness causes pleasure in so far as it is in agreement with the will.
Reply to Objection 3. A man does not strive for mastery in matters where he is very deficient; so that he does not envy one who surpasses him in such matters, unless he surpass him by little, for then it seems to him that this is not beyond him, and so he makes an effort; wherefore, if his effort fails through the other's reputation surpassing his, he grieves. Hence it is that those who love to be honored are more envious; and in like manner the faint-hearted are envious, because all things are great to them, and whatever good may befall another, they reckon that they themselves have been bested in something great. Hence it is written (Job 5:2): "Envy slayeth the little one," and Gregory says (Moral. v, 46) that "we can envy those only whom we think better in some respect than ourselves."
Reply to Objection 4. Recollection of past goods in so far as we have had them, causes pleasure; in so far as we have lost them, causes sorrow; and in so far as others have them, causes envy, because that, above all, seems to belittle our reputation. Hence the Philosopher says (Rhet. ii) that the old envy the young, and those who have spent much in order to get something, envy those who have got it by spending little, because they grieve that they have lost their goods, and that others have acquired goods.
Source: Summa Theologica (New Advent)