In a comment below, Andrew disputes my argument that we can and ought to distinguish between love and obedience as the foundation of the moral order:
It strikes me that, in fact, we ought not make a distinction between love and obedience for this reason. The love to which we are exhorted in the Scriptures is a certain sort of love - agape - which is essentially self-giving love. It is essentially submissive to the will (and needs, when it is directed towards human beings rather than towards God) of the beloved. As St. Paul says, love does not seek its own. Thus we need make no distinction between "love your neighbor" and "serve your neighbor." Just so, we need make no distinction between "Love the Lord your God," and "Obey God," for to love one's neighbor is to seek his good, and to love God is to seek His good. It just so happens that to seek the Good for God is identical with seeking to do His will.
If we make no distinction between "Love God" and "Obey God," then we are incapable of loving God.
We're pretty good, as Catholics, at recognizing that love isn't just an emotion. "'Love' is a verb," as the saying goes.
But love isn't just an action; it's not fundamentally a matter of doing the other's will. It is primarily a matter of being in a relationship of love with the other.
Again, the virtue of charity is friendship with God, friendship in the traditional and profound sense. Friendship can't be commanded. It must be freely given, or it is pretense. A master can't command his servant to be his friend; he can only invite him.
And yet, isn't the first and greatest commandment, "Thou shalt love the Lord thy God, with thy whole heart, and with thy whole soul, and with thy whole mind, and with thy whole strength"?
Yes, and this is further proof that love is the foundation of the moral order! If obedience were the formal cause of this commandment, it would either be impossible to keep or it would refer to some watered-down notion of imperfect love, the kind of love, which is only a matter of doing, that can be commanded. But watered-down, imperfect things don't come from God.
If, on the other hand, love is the formal cause of this commandment, then it is to be understood that our own perfection lies in our perfect love for God. It is love, inviting us to be perfect and instructing us how to become perfect, expressed in the form of a commandment.
Why does God express His invitations as commandments? An incomplete answer: Our relationship with God is one of creature to Creator, of servant to Master, of child to Father. In all of these relationships, when the greater's wish is the lesser's command. When a father says, "I want you to love me," a child ought to hear, "Love me." Not, as I've tried to explain above, as though that is what the father directly commands, but as what their relationship requires of the child, given the father's desire.