Because it is our Creator who gave the word

For what purpose would God command something? It depends very much on who "God" is. If we think of God as a slavemaster and ourselves as his servants, then it is not our place to ask why. If we think of God as a more powerful being who has no special love for us, there may not be a purpose to what he commands. But if God has created us ... then what he tells us to do is part of the same picture as why he made us. What he tells us to do will be the sensible enactment of why we are here. Living that will be the true and ultimate purpose of our lives. Consider this: Would he command something besides fulfilling the purpose for which he made us? To put it another way: if the command came from the Creator, wouldn't the that imply that the command was to live out the purpose for which we exist? If the divine law comes from the Creator, then keeping that law simply means being who we were intended to be, and fulfilling all of our human potential. And it means that straying from it is always cutting short our own potential as humans.