Objection 1. It would seem that sinners love themselves. For that which is the principle of sin, is most of all in the sinner. Now love of self is the principle of sin, since Augustine says (De Civ. Dei xiv, 28) that it "builds up the city of Babylon." Therefore sinners most of all love themselves.
Objection 2. Further, sin does not destroy nature. Now it is in keeping with nature that every man should love himself: wherefore even irrational creatures naturally desire their own good, for instance, the preservation of their being, and so forth. Therefore sinners love themselves.
Objection 3. Further, good is beloved by all, as Dionysius states (Div. Nom. iv). Now many sinners reckon themselves to be good. Therefore many sinners love themselves.
On the contrary, It is written (Psalm 10:6): "He that loveth iniquity, hateth his own soul."
I answer that, Love of self is common to all, in one way; in another way it is proper to the good; in a third way, it is proper to the wicked. For it is common to all for each one to love what he thinks himself to be. Now a man is said to be a thing, in two ways: first, in respect of his substance and nature, and, this way all think themselves to be what they are, that is, composed of a soul and body. On this way too, all men, both good and wicked, love themselves, in so far as they love their own preservation.
Secondly, a man is said to be something in respect of some predominance, as the sovereign of a state is spoken of as being the state, and so, what the sovereign does, the state is said to do. On this way, all do not think themselves to be what they are. For the reasoning mind is the predominant part of man, while the sensitive and corporeal nature takes the second place, the former of which the Apostle calls the "inward man," and the latter, the "outward man" (2 Corinthians 4:16). Now the good look upon their rational nature or the inward man as being the chief thing in them, wherefore in this way they think themselves to be what they are. On the other hand, the wicked reckon their sensitive and corporeal nature, or the outward man, to hold the first place. Wherefore, since they know not themselves aright, they do not love themselves aright, but love what they think themselves to be. But the good know themselves truly, and therefore truly love themselves.
The Philosopher proves this from five things that are proper to friendship. For in the first place, every friend wishes his friend to be and to live; secondly, he desires good things for him; thirdly, he does good things to him; fourthly, he takes pleasure in his company; fifthly, he is of one mind with him, rejoicing and sorrowing in almost the same things. On this way the good love themselves, as to the inward man, because they wish the preservation thereof in its integrity, they desire good things for him, namely spiritual goods, indeed they do their best to obtain them, and they take pleasure in entering into their own hearts, because they find there good thoughts in the present, the memory of past good, and the hope of future good, all of which are sources of pleasure. Likewise they experience no clashing of wills, since their whole soul tends to one thing.
On the other hand, the wicked have no wish to be preserved in the integrity of the inward man, nor do they desire spiritual goods for him, nor do they work for that end, nor do they take pleasure in their own company by entering into their own hearts, because whatever they find there, present, past and future, is evil and horrible; nor do they agree with themselves, on account of the gnawings of conscience, according to Psalm 49:21: "I will reprove thee and set before thy face."
In the same manner it may be shown that the wicked love themselves, as regards the corruption of the outward man, whereas the good do not love themselves thus.
Reply to Objection 1. The love of self which is the principle of sin is that which is proper to the wicked, and reaches "to the contempt of God," as stated in the passage quoted, because the wicked so desire external goods as to despise spiritual goods.
Reply to Objection 2. Although natural love is not altogether forfeited by wicked men, yet it is perverted in them, as explained above.
Reply to Objection 3. The wicked have some share of self-love, in so far as they think themselves good. Yet such love of self is not true but apparent: and even this is not possible in those who are very wicked.
Source: Summa Theologica (New Advent)