Objection 1. It would seem that the order of charity is not included in the precept. For whoever transgresses a precept does a wrong. But if man loves some one as much as he ought, and loves any other man more, he wrongs no man. Therefore he does not transgress the precept. Therefore the order of charity is not included in the precept.
Objection 2. Further, whatever is a matter of precept is sufficiently delivered to us in Holy Writ. Now the order of charity which was given above (Article 26) is nowhere indicated in Holy Writ. Therefore it is not included in the precept.
Objection 3. Further, order implies some kind of distinction. But the love of our neighbor is prescribed without any distinction, in the words, "Thou shalt love thy neighbor as thyself." Therefore the order of charity is not included in the precept.
On the contrary, Whatever God works in us by His grace, He teaches us first of all by His Law, according to Jeremiah 31:33: "I will give My Law in their heart [Vulgate: 'in their bowels, and I will write it in their heart']." Now God causes in us the order of charity, according to Canticles 2:4: "He set in order charity in me." Therefore the order of charity comes under the precept of the Law.
I answer that, As stated above (4, ad 1), the mode which is essential to an act of virtue comes under the precept which prescribes that virtuous act. Now the order of charity is essential to the virtue, since it is based on the proportion of love to the thing beloved, as shown above (25, 12; 26, 1 and 2). It is therefore evident that the order of charity must come under the precept.
Reply to Objection 1. A man gratifies more the person he loves more, so that if he loved less one whom he ought to love more, he would wish to gratify more one whom he ought to gratify less, and so he would do an injustice to the one he ought to love more.
Reply to Objection 2. The order of those four things we have to love out of charity is expressed in Holy Writ. For when we are commanded to love God with our "whole heart," we are given to understand that we must love Him above all things. When we are commanded to love our neighbor "as ourselves," the love of self is set before love of our neighbor. On like manner where we are commanded (1 John 3:16) "to lay down our souls," i.e. the life of our bodies, "for the brethren," we are given to understand that a man ought to love his neighbor more than his own body; and again when we are commanded (Galatians 6:10) to "work good... especially to those who are of the household of the faith," and when a man is blamed (1 Timothy 5:8) if he "have not care of his own, and especially of those of his house," it means that we ought to love most those of our neighbors who are more virtuous or more closely united to us.
Reply to Objection 3. It follows from the very words, "Thou shalt love thy neighbor" that those who are nearer to us are to be loved more.
Source: Summa Theologica (New Advent)