"This is... the first commandment. The second is like it"
Since we are directed to love our neighbor as ourselves, let us consider whether we have received from the Lord the power to fulfill this commandment also… Nothing, indeed, is so compatible with our nature as living in society and in dependence upon one another and as loving our own kind. Now, the Lord himself gave to us the seeds of these qualities in anticipation of his requiring in due time their fruits, for he says: “A new commandment I give to you: that you love one another” (Jn 13, 34).
Moreover, wishing to animate our soul to the observance this commandment, he did not require signs or wonders the means of recognizing his disciples (although He gave power of working these also in the Holy Spirit), but he says: “By this shall everyone know you are my disciples, if you have love one for another” (Jn 13,35). Further, he establishes so close a connection between the two great commandments that benefits conferred upon the neighbor are transferred to himself: “For I was hungry,” he says, “and you gave me to eat” and so on, adding: “as long as you did it to one of these the least of my brethren, you did it to me” (Mt 25,35-40).
It is, accordingly, possible to keep the second commandment by observing the first, and by means of the second we are led back to the first. Whoever loves the Lord loves their neighbor in consequence. “If anyone love me,” said the Lord, “he will keep my commandments; this is my commandment, that you love one another as I have loved you” (Jn 14,23; 15,12). On the other hand, whoever loves a neighbor fulfills the love owed to God, for God accepts this favor as shown to himself.
St. Thomas Aquinas