The answer to the question, "Why"?" directly reflects our hearts. It's actually the way that God prefers for us to follow Him. He didn't intend for us to blindly follow His laws, but instead wants our motivation to be intrinsic. In Psalm 40:8 David writes, "I desire to do your will, my God; your law is within my heart."
This attitude reveals the true impetus for godly behavioral change. It's not based on the law (no matter how good that law might be for us) but is instead a result of a yearning to do God's will. If we don't shape our acceptance of God's direction in our hearts we won't really pursue change. We will simply turn back to old patterns of behavior when the truth is no longer convenient for us.
Our focus on our own "why" breeds loyalty and devotion and shapes our morals to honor God. It's the internal commitment that endures.