Patristic and medieval Christianity was heavily influenced by the eudaimenism of the ancient Greeks. The main way that the Greeks conceptualized the human person was as a desiring subject. As a desiring subject, the question of ethics is whether or not the desiring subject desires the right sorts of things. Namely, temporal or eternal- spiritual or earthly?
In medieval Christianity from Augustine onward this is projected onto God. God is the supreme being and goodness, therefore he desires being and goodness. He's hungry for it in a sense. As Aquinas puts it (following both Aristotle and Augustine) God comprehends and enjoys himself in his Son, the divine intellect. The love of God's intellect for himself and his love for his intellect is Holy Spirit. Father, Son, and Holy Spirit are Desirer, Desirered, Desiring.
We can observe how this relates to the doctrine of grace as well. To be loved is to be desirable. God can't love things that aren't like him in his goodness and being. Therefore the goal of all grace is to make us desirable- i.e. our moral renovation. Also, we can see why for the Roman Catholic it is love and not faith that justifies. Love is right desire- it is to be like God, because God as the supreme good desires himself. Consequently, it also makes us desirable to the supreme desirer.
Enter Ockham. Ockham rejects Realist ontology of Aquinas and Augustine and their concept of God as the supreme good. Rather, God is a willing subject. He doesn't do things because he's the supreme good and the supreme good just does good things because he has to- that is it's part of his nature. He does things becaue he agrees to do them in the form of a "pactum" or covenant.
This entirely shifts the doctrine of God. God is no longer conceived primarily a desiring subject, but a speaking or promising subject. How do I get saved? By doing what is within me. Why is this a valid path to salvation (according to Ockham) is becuase God promised it is! There's nothing inherently attractive about your works or your love. God's has simply decreed and promised that they are valid before him.
I think that this then explains why Luther can think of divine agency in the way that he does. God is, following his master Ockham and Biel, a speaking subject. God is always speaking through the mediums of the created order. He speaks words of either law or gospel. The creature then is considered inherently passive in relationship to God. He (to use Bayer's term) "suffers" God's words. In the sphere of law, the human subject's flesh is restrained by the created orders and his conscience is driven to despair and/or repentance. In the sphere of the gospel, he is justified and sanctified by the promise of justitification.
In the end, if God is a speaking subject, one can simply passive receive what he says. All the desire flows from one's determination as a creature- which in turn results from suffering God's Word in its various forms.
This also explains Luther's Trinitarian metaphors in his later disputations on that subject. According to the old Luther, Father, Son and Holy Spirit are "speaker, speaking, hearing." Again, God as the speaking, promising subject is defined by his Word, not his abstract desiring as the supreme good.