True love is born in man when he recognizes the fitness of the object, however. We love because we perceive the value, not because we're obliged. Casuistry changed charity into the obligation to love God and our neighbor. Saint Thomas doesn't make obligation the center of the moral order. He doesn't treat it as a principle which defends itself.
Obligations find their center in the finality of man.
The objectiveness of the objects toward which we move obliges us. Friendship places some demands, but the source of the obligation is the desire for happiness which seeks friends. Obligation comes from something which is good attracting the will. We have within us a natural inclination to goodness. The fact that we perceive the goodness is tying.