Objection 1. It would seem that man is not bound, out of charity, to love God more than himself. For the Philosopher says (Ethic. ix, 8) that "a man's friendly relations with others arise from his friendly relations with himself." Now the cause is stronger than its effect. Therefore man's friendship towards himself is greater than his friendship for anyone else. Therefore he ought to love himself more than God.
Objection 2. Further, one loves a thing in so far as it is one's own good. Now the reason for loving a thing is more loved than the thing itself which is loved for that reason, even as the principles which are the reason for knowing a thing are more known. Therefore man loves himself more than any other good loved by him. Therefore he does not love God more than himself.
Objection 3. Further, a man loves God as much as he loves to enjoy God. But a man loves himself as much as he loves to enjoy God; since this is the highest good a man can wish for himself. Therefore man is not bound, out of charity, to love God more than himself.
On the contrary, Augustine says (De Doctr. Christ. i, 22): "If thou oughtest to love thyself, not for thy own sake, but for the sake of Him in Whom is the rightest end of thy love, let no other man take offense if him also thou lovest for God's sake." Now "the cause of a thing being such is yet more so." Therefore man ought to love God more than himself.
I answer that, The good we receive from God is twofold, the good of nature, and the good of grace. Now the fellowship of natural goods bestowed on us by God is the foundation of natural love, in virtue of which not only man, so long as his nature remains unimpaired, loves God above all things and more than himself, but also every single creature, each in its own way, i.e. either by an intellectual, or by a rational, or by an animal, or at least by a natural love, as stones do, for instance, and other things bereft of knowledge, because each part naturally loves the common good of the whole more than its own particular good. This is evidenced by its operation, since the principal inclination of each part is towards common action conducive to the good of the whole. It may also be seen in civic virtues whereby sometimes the citizens suffer damage even to their own property and persons for the sake of the common good. Wherefore much more is this realized with regard to the friendship of charity which is based on the fellowship of the gifts of grace.
Therefore man ought, out of charity, to love God, Who is the common good of all, more than himself: since happiness is in God as in the universal and fountain principle of all who are able to have a share of that happiness.
Reply to Objection 1. The Philosopher is speaking of friendly relations towards another person in whom the good, which is the object of friendship, resides in some restricted way; and not of friendly relations with another in whom the aforesaid good resides in totality.
Reply to Objection 2. The part does indeed love the good of the whole, as becomes a part, not however so as to refer the good of the whole to itself, but rather itself to the good of the whole.
Reply to Objection 3. That a man wishes to enjoy God pertains to that love of God which is love of concupiscence. Now we love God with the love of friendship more than with the love of concupiscence, because the Divine good is greater in itself, than our share of good in enjoying Him. Hence, out of charity, man simply loves God more than himself.
Source: Summa Theologica (New Advent)