St. Thomas begins his study of morality, not with the question, "What ought man do?," but with, "Why does anyone do anything?" Though he's writing as a teacher of beginning students of sacra doctrina, his approach is also pastorally congenial, as I suggested below.
Of course, people desire all sorts of things, good and bad, many of which are mutually incompatible. To achieve happiness, then, isn't merely a matter of obtaining everything you might happen to desire. You first have to make sure that everything you desire can be completely attained.
This fact is a big reason virtue-based morality -- expressing what man ought do in terms of virtues (good habits we should cultivate) and vices (bad habits we should eliminate) -- is preferable to rule-based morality -- expressing what man ought to do in terms of proscriptions and prescriptions. You can follow all the rules and still not be happy.
Which is not to say proscriptions and prescriptions are unimportant, but that they work neither as a starting point nor as an ending point if the human moral life is to flourish. As Fr. Romanus Cessario, OP, puts it in A Short History of Thomism [p. 22]:
In moral philosophy, Thomists agree that by nature man enjoys the right to dwell in community and to pursue personal happiness within the common good, and that the right conduct of human beings is best described by appeal to the virtues of human life, although laws, both natural and positive, also legitimately direct human action.