The law of nature requires that all be done from love of God above all
Thesis XIII. That no work can be truly good by the law of nature unless it flows from the love of God, they prove from the fact that this law of nature requires man to love God his creator, from whose goodness he has all that he is and all that he can do, with all his strength and above all things.
Whoever is moved to act by love of creatures alone, and not by love of God, shows by this that he does not love God above all creatures. He prefers love of the creature to love of God, and so he acts viciously.
Latin
XIII. Et quidem quod nullum opus, secundum naturae legem, vere bonum esse possit, nisi fluat ex Dei amore, inde probant, quod illa naturae lex ab homine exigit, ut Deum creatorem suum, a cujus bonitate habet quicquid est, & potest, totis viribus & supra omnia diligat. Qui autem ex solo creaturarum amore, non vero ex amore Dei ad agendum movetur, in eo testatur se Deum supra creaturas omnes non diligere, sed creaturae amorem amori divino praeferre, ac proinde vitiose agit.