2. Whoever loves another, must in consequence also love those whom that other loves and who are united with him.* But men are loved by God, seeing that for them He has prepared the enjoyment of Himself as their last end. Therefore as one is a lover of God, so must he also be a lover of his neighbour.
3. Since man is naturally a social animal, he needs to be helped by other men to gain his proper end; and this is most aptly done by mutual love prevailing amongst men.
4. To attend to divine things, a man needs tranquillity and peace. Now the things that might trouble peace are most effectually taken away by mutual love. Since then the law of God orders men to attend to divine things, mutual love amongst men must necessarily be a provision of the divine law.
5. The divine law is given to man to bear out the natural law.* But it is natural to all men to love one another: a sign of this is the fact that by a sort of natural instinct man helps any man, even a stranger, in necessity, as by calling him back from a wrong turn that he may have taken on his way, lifting him up from a fall, and the like, as though every man were kinsman and friend of every other man.
Hence it is said: This is my commandment, that ye love one another (John xv, 12): This commandment we have of God, that he who loveth God do love also his brother (1 John iv, 21): The second commandment is, thou shalt love thy neighbour as thyself (Matt. xxii, 39).
3.116 : That the End of the Divine Law is the Love of God
3.118 : That by Divine Law men are obliged to a Right Faith