We tend to think of the law as limiting, for good reasons of course, but restrictive none-the-less. The psalmist sees the law as the kind of freedom that revives, rejoices and enlightens. That is because the law of the Lord reveals the truth about the One who desires all good things for us including clearing us from hidden faults, which is not all that pleasant for us (there is a reason our faults prefer to remain hidden) but it is certainly a good thing to know oneself well enough to avoid being dominated by errors that are not easily detected. So when we embody the perfect law of the Lord we live the love of the Lord which Jesus said is what the law and the prophets are all about. Love God. Love others. Such words and thoughts of the heart are always pleasing to the Lord, our rock and redeemer.