Whatever God requires of us is included in a precept. Now God requires that man should love Him, according to Dt. 10:12. Therefore it behooved precepts to be given about the love of charity, which is the love of God.In other words, God tells us what it means to love him: what we must do if we love him. It's not enough just to say we love someone: our actions must demonstrate it. This is pre-eminently so with God, whom we ought to love above all others. And this is completely in keeping with what he tells us.
- If you love me, keep my commandments.
- He who has my commandments and keeps them, he it is who loves me.
- If anyone love me, he will keep my word, and my Father will love him, and we will come to him and make our abode with him.
- Now the works of the flesh are manifest, which are immorality, uncleanness, licentiousness, idolatry, witchcrafts, enmities, contentions, jealousies, anger, quarrels, factions, parties, envies, murders, drunkenness, carousings, and suchlike. And concerning these I warn you, as I have warned you, that they who do such things will not attain the kingdom of God.
- Hear, O Israel! The Lord our God, the Lord is one! Therefore you shall love the Lord your God with all your heart, and with all your soul, and with all your strength. Take to heart these words which I enjoin on you today.
(John 14:15, 21, 23; Galatians 5:19-21; Deuteronomy 6:4-5)Because what St. Thomas says on this subject is so good, I can hardly do better than to quote him at some length here.
[A] precept implies the notion of something due. Hence a thing is a matter of precept, in so far as it is something due. Now a thing is due in two ways, for its own sake, and for the sake of something else. On every affair, it is the end that is due for its own sake, because it has the character of a good for its own sake: while that which is directed to the end is due for the sake of something else: thus for a physician, it is due for its own sake, that he should heal, while it is due for the sake of something else that he should give a medicine in order to heal. Now the end of the spiritual life is that man be united to God, and this union is effected by charity, while all things pertaining to the spiritual life are ordained to this union, as to their end. Hence the Apostle says (1 Timothy 1:5): "The end of the commandment is charity from a pure heart, and a good conscience, and an unfeigned faith." For all the virtues, about whose acts the precepts are given, are directed either to the freeing of the heart from the whirl of the passions--such are the virtues that regulate the passions--or at least to the possession of a good conscience--such are the virtues that regulate operations--or to the having of a right faith--such are those which pertain to the worship of God: and these three things are required of man that he may love God. For an impure heart is withdrawn from loving God, on account of the passion that inclines it to earthly things; an evil conscience gives man a horror for God's justice, through fear of His punishments; and an untrue faith draws man's affections to an untrue representation of God, and separates him from the truth of God. Now in every genus that which is for its own sake takes precedence of that which is for the sake of another, wherefore the greatest precept is that of charity, as stated in Mt. 22:39.So the virtues are intended and given to us in order that we may be freed to love God, and that we may more readily order our lives according to that love.
But this duty to love God (and neighbor, for that matter) is not something that ought to be viewed as a burden:
The obligation of a precept is not opposed to liberty, except in one whose mind is averted from that which is prescribed, as may be seen in those who keep the precepts through fear alone. But the precept of love cannot be fulfilled save of one's own will, wherefore it is not opposed to charity.If you love God, you will keep his commandments: this is something we do because we love him, and that we must do freely (or else it is not really love).