Objection 1. It would seem that self-love is not the source of every sin. For that which is good and right in itself is not the proper cause of sin. Now love of self is a good and right thing in itself: wherefore man is commanded to love his neighbor as himself (Leviticus 19:18). Therefore self-love cannot be the proper cause of sin.
Objection 2. Further, the Apostle says (Romans 7:8): "Sin taking occasion by the commandment wrought in me all manner of concupiscence"; on which words a gloss says that "the law is good, since by forbidding concupiscence, it forbids all evils," the reason for which is that concupiscence is the cause of every sin. Now concupiscence is a distinct passion from love, as stated above (3, 2; 23, 4). Therefore self-love is not the cause of every sin.
Objection 3. Further, Augustine in commenting on Psalm 79:17, "Things set on fire and dug down," says that "every sin is due either to love arousing us to undue ardor or to fear inducing false humility." Therefore self-love is not the only cause of sin.
Objection 4. Further, as man sins at times through inordinate love of self, so does he sometimes through inordinate love of his neighbor. Therefore self-love is not the cause of every sin.
On the contrary, Augustine says (De Civ. Dei xiv, 28) that "self-love, amounting to contempt of God, builds up the city of Babylon." Now every sin makes man a citizen of Babylon. Therefore self-love is the cause of every sin.
I answer that, As stated above (Question 75, Article 1), the proper and direct cause of sin is to be considered on the part of the adherence to a mutable good; in which respect every sinful act proceeds from inordinate desire for some temporal good. Now the fact that anyone desires a temporal good inordinately, is due to the fact that he loves himself inordinately; for to wish anyone some good is to love him. Therefore it is evident that inordinate love of self is the cause of every sin.
Reply to Objection 1. Well ordered self-love, whereby man desires a fitting good for himself, is right and natural; but it is inordinate self-love, leading to contempt of God, that Augustine (De Civ. Dei xiv, 28) reckons to be the cause of sin.
Reply to Objection 2. Concupiscence, whereby a man desires good for himself, is reduced to self-love as to its cause, as stated.
Reply to Objection 3. Man is said to love both the good he desires for himself, and himself to whom he desires it. Love, in so far as it is directed to the object of desire (e.g. a man is said to love wine or money) admits, as its cause, fear which pertains to avoidance of evil: for every sin arises either from inordinate desire for some good, or from inordinate avoidance of some evil. But each of these is reduced to self-love, since it is through loving himself that man either desires good things, or avoids evil things.
Reply to Objection 4. A friend is like another self (Ethic. ix): wherefore the sin which is committed through love for a friend, seems to be committed through self-love.
Source: Summa Theologica (New Advent)