I take joy in doing your will, my God, for your instructions are written on my heart.
Undeniably, there are times when obeying God is hard, times when we'd much rather follow our own will than his. Even Jesus knew that experience as he wrestled with his Father's will in the Garden of Gethsemane.
Yet there are also times when doing God's will is easy, even delightful. In Psalm 40:8 we read, "I take joy in doing your will, my God." The sense of the Hebrew verb translated here as "I take joy" is experiencing pleasure. We receive pleasure from knowing that we are doing that which honors and glorifies God. Moreover, when we do what's right, we feel a sense of well-being that comes from achieving our life's purpose. We have been put on this earth to live for God's glory. When we do it, we feel joy.
In Psalm 40:8, David explains his delight in obeying God by adding "for your instructions are written on my heart." The Hebrew word translated as "heart" literally means "belly" or "bowels." It was used much as we speak of the heart, as the physical location of emotions. In this verse, David is not saying only that his thinking has been shaped by God's law, but also that his emotions have been honed by God's instructions. Thus he feels happy when he acts according to God's will.
I would love to get to the place in life where I truly delight in doing what pleases the Lord, even those things that are not easy for me right now.