Objection 1. It would seem that the precept of the love of our neighbor is unfittingly expressed. For the love of charity extends to all men, even to our enemies, as may be seen in Matthew 5:44. But the word "neighbor" denotes a kind of "nighness" which does not seem to exist towards all men. Therefore it seems that this precept is unfittingly expressed.
Objection 2. Further, according to the Philosopher (Ethic. ix, 8) "the origin of our friendly relations with others lies in our relation to ourselves," whence it seems to follow that love of self is the origin of one's love for one's neighbor. Now the principle is greater than that which results from it. Therefore man ought not to love his neighbor as himself.
Objection 3. Further, man loves himself, but not his neighbor, naturally. Therefore it is unfitting that he should be commanded to love his neighbor as himself.
On the contrary, It is written (Matthew 22:39): "The second" commandment "is like to this: Thou shalt love thy neighbor as thyself."
I answer that, This precept is fittingly expressed, for it indicates both the reason for loving and the mode of love. The reason for loving is indicated in the word "neighbor," because the reason why we ought to love others out of charity is because they are nigh to us, both as to the natural image of God, and as to the capacity for glory. Nor does it matter whether we say "neighbor," or "brother" according to 1 John 4:21, or "friend," according to Leviticus 19:18, because all these words express the same affinity.
The mode of love is indicated in the words "as thyself." This does not mean that a man must love his neighbor equally as himself, but in like manner as himself, and this in three ways. First, as regards the end, namely, that he should love his neighbor for God's sake, even as he loves himself for God's sake, so that his love for his neighbor is a "holy" love. Secondly, as regards the rule of love, namely, that a man should not give way to his neighbor in evil, but only in good things, even as he ought to gratify his will in good things alone, so that his love for his neighbor may be a "righteous" love. Thirdly, as regards the reason for loving, namely, that a man should love his neighbor, not for his own profit, or pleasure, but in the sense of wishing his neighbor well, even as he wishes himself well, so that his love for his neighbor may be a "true" love: since when a man loves his neighbor for his own profit or pleasure, he does not love his neighbor truly, but loves himself.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)