See how his thinking works? He's writing a textbook on sacra doctrina, on the science of divine revelation, and so begins naturally enough with its object -- viz, God. Man first enters into the discussion as a special feature of God's creation.
The specialest feature of man is his free will, and how he does and ought to use it is basically the subject of the whole Secunda Partis. The thing is, St. Thomas begins this discussion with the question of why man uses his free will. To the fundamental question of moral theology, "What must I do?," St. Thomas replies, "That depends on what you want." Seven articles in, St. Thomas recalls the words of St. Augustine:
But if he had said, You all will to be blessed, you do not will to be wretched; he would have said something which there is no one that would not recognize in his own will. For whatever else a man may will secretly, he does not withdraw from that will, which is well known to all men, and well known to be in all men.
The remaining 303 questions in this part of the Summa look at what "to be blessed" means and how to achieve it.
Note how natural, human, and congenial this approach to moral theology is. Natural, because it situates moral theology in its proper place within the whole of sacra doctrina*. Human, because it considers the human act of moral choice as it is in itself, an act of free will directed by reason. Congenial, because it begins by asking what you want, and everyone, of whatever age and whatever spiritual stage, wants to be happy.
* I use "sacra doctrina" not merely to be pretentious, but because I've been told it's a tricky term to translate to English. "Sacred doctrine," "holy teaching," and suchlike evidently don't quite capture the full scope of the term as St. Thomas used it, and as early as the first article of the first question of the first part of the Summa Theologiae, St. Thomas distinguishes between sacra doctrina and theology.