Psalm 19 has the following wonderful passage:
The law of the Lord is perfect,
reviving the soul;
the decrees of the Lord are sure,
making wise the simple;
the precepts of the Lord are right,
rejoicing the heart;
the commandment of the Lord is clear,
enlightening the eyes;
the fear of the Lord is pure,
the ordinances of the Lord are true
and righteous altogether.
More to be desired are they than gold,
even much fine gold;
sweeter also than honey,
and drippings of the honeycomb (vv. 7-10).
For some Christians, it may be incredibly difficult to understand these sentiments. How can a law like God's be perfect, reviving the soul? How can they be worth more than gold and sweeter than honey? Not a few persons are convinced that God is merely interested in forbidding them from doing the things they love, in keeping them from having any fun. How can it be good and perfect if you aren't allowed to enjoy yourself?
There are quite a few things wrong with that attitude, and it is not something that can be taught or trained out of a person in a few short minutes or hours or even days. The impulse to give in to our irrational impulses and live a life in pursuit of the pleasures of the moment is strong in every one of us, I think, even if we may not all give in to this impulse in equal measure. It is true that we may inevitably and ultimately seek enjoyment in what we do, but the question ought to be asked: might it be that the things we think will give us quick and easy enjoyment are not actually worth doing? And might it be that God's laws will bring enjoyment of a greater, lasting kind if only we would keep them with diligence?
I subscribe to what is called a natural law theory of ethics. I think right and wrong, good and evil are determined in accordance with the nature we have as human beings. We have certain natural impulses or goals that we seek to obtain, and the frustration of these impulses or goals is evil, as is obtaining them in an improper and unnatural way, whereas their satisfaction or obtainment is good. The natural law theory of Christian ethics teaches that God's laws are designed so that we fulfill the drives and goals we naturally have in the proper way.
Thomas Aquinas was perhaps the greatest of the natural ethicists, and he dedicates a lot of time and space, and spills very much ink, demonstrating how God's laws and a life of virtue are the fulfillment of our nature as humans. Consider, for instance, the many questions and articles of his Summa theologica! In all his answers, he attempts to demonstrate how our nature, or more specifically some impulse or drive that we have by nature, is fulfilled by obedience to God's law.
I think a more sophisticated understanding of God's laws in terms of the natural law is important for any Christian to understand. It's important because it helps us to see how God's laws make sense, how the Christian church's teaching is rational and reasonable. The logic may not be initially obvious, but upon study it becomes clearer and clearer. (The truth is that nothing is really at that clear or obvious without much study and dedication; it's foolish to think that we would just see the truth or that it would be obvious to us in these matters, with perhaps a few exceptions.)
I'll give one example. Why shouldn't a young Christian guy and his girlfriend engage in sex before they are married? They love each other, they don't plan on cheating on each other or taking advantage of one another -- what is the problem?
Thomas Aquinas would reason thus (compare to this discussion here, ST IIa IIae, q. 154, art. 2). It is obvious that the function of the sexual organs we have by nature is procreation: that is the result when they are made use of without unnatural interruptions (e.g., the use of contraceptives). But procreation demands commitment on the part of both partners: the woman is committed to having the child which has come into being inside her, and the man is committed to assisting the woman whom he has impregnated. And when that child is born, it is entirely helpless and in need of the presence of both parents for a normal and healthy upbringing. Marriage is what we call the proper context for the upbringing of children: two partners committed to one another and to taking care of the child which comes from their union. This means that sexuality is naturally paired with marriage, and to have sex outside of the context of marriage is to live outside of the direction that nature gives us. It is to go against nature and to attempt to live apart from its natural course. (The same is true of sex with the use of contraceptives, as well as masturbation, gay sex, etc., which is why these too are also immoral.) In this way, the law against sex outside of marriage makes sense and it is reasonable.
I'll also add one further point: to try to tinker with this natural law line of reasoning in one way or another is to compromise the rationality of the law. If it's not wrong to have sex with the use of contraceptives, if sex can be separated from bearing and rearing children -- as it is in the minds of many people these days -- then there is no obvious reason at all why it should be immoral to have sex outside of marriage. So I think the natural law theory of ethics is especially important for this reason: the laws of God quickly become unmotivated and apparently arbitrary apart from this line of thinking.