“While it desires the Eternal Good and measures its wish for secondary goods in reason, this love cannot give rise to sinful pleasures. But when it turns to evil, or shows more or less zeal than it ought for what is good, then the creature turns on its Creator. Thus you may understand that love alone is the true seed of every merit in you, and of all its acts for which you must atone.”
What packed lines. I will hardly do justice to a few of the points made here, such as the implied relation between reason, truth, and the intellect and the will and the good.
Love is the cause of both merit and of that for which we must make satisfaction? Yes, because we are moved of our will toward the good. This much in our will is in fact determined, that good is that which it seeks.
“The will is a rational appetite. Now every appetite is only of something good. The reason of this is that the appetite is nothing else than an inclination of a person desirous of a thing towards that thing. Now every inclination is to something like and suitable to the thing inclined. Since, therefore, everything, inasmuch as it is being and substance, is a good, it must needs be that every inclination is to something good. And hence it is that the Philosopher says (Ethic. i. 1) that the good is that which all desire.” (STh., I-II q.8 a.1 resp.)
We are not determined to one particular good over another particular good, however much we may be determined to good in itself. In this is our free choice that includes a “freedom” to sin. Of course, true freedom is bound up in truth, but God’s calling for us to freely love Him entails our ability to reject this love. We may ignore the directive of the Apostle John when he says “Little children, keep yourselves from idols,” (1 Jn 5:21), but when we “show more or less zeal than we ought” for certain goods or the good itself, this is exactly what we do. We take God’s good creation and make an idol of individual particular parts of it, and then we place them between ourselves and God, losing sight of Him for a good He made.
Virgil later wraps this up, poetically saying “All men, though in a vague way, apprehend a good their souls may rest in, and desire it; each, therefore, strives to reach his chosen end.”