The Apostle Paul is one of the most important writers of letters and epistles to the churches, one of them is addressed to the community of Rome. In it, he explains fundamentally about Justification, the divine law and the sin. The doctrine of Justification exposed in this letter will influence the differences between Catholics and Protestants. and the ideas of Luther.
One of the most important aspects for the Apostle is the difference between the flesh and the spirit. There is a law of the flesh and another of the spirit: I do not do the good that I want to do, but the evil I do not want to do, Romans 7:19. To which obey?
To understand the Biblical assertion is necessary analyze the passions and the attachments. Aristotle argues in one of his works that passions are those things that accompany the pain and the gladness. Given the definition of passions, some are: shame, guilt, anger, fear, love, joy, hope. You have to understand this definition of passion from the triple division of the soul of Plato: passion, reason and desire. Then, and following this reasoning we have passions oriented to the good: as charity, joy and hope; and passions oriented to evil as anger, fear, guilt.
The scholastic and Christian thought in general make a change, and this change is from the triple division of man: body, soul and spirit. From this division of man, the concept of passion changed and a new concept arises: the attachment.
From this point of view the passions are psychic movements of the soul, the passions of the soul are not subject to reason and many times can prevent or impede the reasoning. From these the Biblical affirmation. In this scheme God loves without passion, because God is spirit.
The  attachments instead accompanies the will and perfect it, like the Charity, the joy and the hope. Never the attachments are oppose to the will. In God there are no passions but attachments or affections. Love can be considered a passion understanding Love as a genre: Eros and Philia. But Agape, the love of charity, love for the good is not. The passions would respond to the flesh and the attachments to the "law of the spirit" thence the difference between the good that is wanted and is not done.