Objection 1. It would seem that there should not have been given two precepts of charity. For the precepts of the Law are directed to virtue, as stated above (1, Objection 3). Now charity is one virtue, as shown above (Question 33, Article 5). Therefore only one precept of charity should have been given.
Objection 2. Further, as Augustine says (De Doctr. Christ. i, 22,27), charity loves none but God in our neighbor. Now we are sufficiently directed to love God by the precept, "Thou shalt love the Lord thy God." Therefore there was no need to add the precept about loving our neighbor.
Objection 3. Further, different sins are opposed to different precepts. But it is not a sin to put aside the love of our neighbor, provided we put not aside the love of God; indeed, it is written (Luke 15:26): "If any man come to Me, and hate not his father, and mother... he cannot be My disciple." Therefore the precept of the love of God is not distinct from the precept of the love of our neighbor.
Objection 4. Further, the Apostle says (Romans 13:8): "He that loveth his neighbor hath fulfilled the Law." But a law is not fulfilled unless all its precepts be observed. Therefore all the precepts are included in the love of our neighbor: and consequently the one precept of the love of our neighbor suffices. Therefore there should not be two precepts of charity.
On the contrary, It is written (1 John 4:21): "This commandment we have from God, that he who loveth God, love also his brother."
I answer that, As stated above (I-II, 91, 3; I-II, 94, 2) when we were treating of the commandments, the precepts are to the Law what propositions are to speculative sciences, for in these latter, the conclusions are virtually contained in the first principles. Hence whoever knows the principles as to their entire virtual extent has no need to have the conclusions put separately before him. Since, however, some who know the principles are unable to consider all that is virtually contained therein, it is necessary, for their sake, that scientific conclusions should be traced to their principles. Now in practical matters wherein the precepts of the Law direct us, the end has the character of principle, as stated above (23, 07, ad 2; 26, 1, ad 1): and the love of God is the end to which the love of our neighbor is directed. Therefore it behooved us to receive precepts not only of the love of God but also of the love of our neighbor, on account of those who are less intelligent, who do not easily understand that one of these precepts is included in the other.
Reply to Objection 1. Although charity is one virtue, yet it has two acts, one of which is directed to the other as to its end. Now precepts are given about acts of virtue, and so there had to be several precepts of charity.
Reply to Objection 2. God is loved in our neighbor, as the end is loved in that which is directed to the end; and yet there was need for an explicit precept about both, for the reason given above.
Reply to Objection 3. The means derive their goodness from their relation to the end, and accordingly aversion from the means derives its malice from the same source and from no other
Reply to Objection 4. Love of our neighbor includes love of God, as the end is included in the means, and vice versa: and yet it behooved each precept to be given explicitly, for the reason given above.
Source: Summa Theologica (New Advent)