The purpose of studying ethics is to pursue the good life. Most of the classical and medieval philosophers understood this, as ethical systems developed from virtue theories to traditional natural law. Of course, there were those moral outliers – the hedonists, say. Just as there are moral outliers today, though they are becoming more mainstream, I’m afraid: consequentialists, relativists, and so on.
For me, studying ethics is about studying human nature and the perfection of it. Therefore, ethics cannot itself be entirely separated from the greater metaphysical questions or even philosophical anthropology. To speak of “the good” of something requires at least some understanding of essences or substantial forms, and of some higher organizing, indeed I would argue intentionalizing, principle behind them: the highest good, or more simply The Good Itself, which is God.
But then we must reflect on human nature specifically. What are we and what is unique about us. Again, traditionally, reason and will have received considerable attention. While we share many aspects with the lower animals, including sexual appetites and hunger and so on, which are fine and good and meant themselves to be appropriately satisfied, it is our capacity to reason, to hunt down truth, that marks us off most decidedly from the baboon. Reason is our highest principle, what makes us distinctly a certain kind of animal: traditionally, a rational animal.
To flourish then, we must act according to reason, we must discern the truth about ourselves and what perfects us. Truth perfects reason, just as nutrition perfects eating, or procreation sex. Because of this, reason must order our other passions and appetites – put them in their proper place, as it were (such as sex in the context of marriage, eating in the context of health/socialization, etc) – because when we subvert reason, we either significantly limit or frustrate our flourishing: we harm ourselves, whether we initially realize it or not. Reason not only makes possible present goods, but goods of history and goods of the future. It also makes possible goods of the family, goods of the community, and spiritual goods, as well.
Reason is how we can come to understand our human nature, the virtues which perfect it, and how best to pursue our highest and ultimate end, which is eternal communion with God.
PS – It is precisely because we can reason — more specifically, because we can entertain abstract concepts and choose between particular finite goods in pursuit of some higher good — that morality is possible. It is because we can choose to focus on limited goods at the expense (or at our of proportion) to higher goods, that we cause ourselves to sin/act immorally. Beings without reason are not moral agents. Reason is what makes ethics possible.