Why do men have to obey God's Laws? The reason is because this whole world is created by God. He is the Master Designer, and the only one who knows all the operation principles in this universe.
Each and every species has their own set of rules
There are four seasons in a year, the sun rises every morning, cattle and sheep feed on grass, and fish live in water ..... Each and every species has their own set of rules. If one day the sun suddenly decides to rise at night instead of morning, wouldn't the world be in chaos?
Since the creation of the heavens and the earth, the eternal power and divinity of God has been clearly known, though invisible to the eyes, but through the things that have been made, it is known, so that there is no excuse. Romans 1:20
The world is a very precise, subtle and perfectly formulated structure
All things are inter-related. Everything depends on one another for survival, and serves one another in return. Bees disperse pollen for plants, plants produce fruits for human consumption, and humans till the ground and take care of the cattle and plants.
Mankind is the only creation that has been given FREE WILL
Every creation follows the Laws of Nature and does not require any instructions by anyone. Mankind is the only creation that has been given free will by God. However, because of this freedom, many problems have also arisen as a result. As it is said in the Book of Jeremiah:
Even the stork in the sky knows her seasons (of migration), And the turtledove, the swallow and the crane observe the time of their return. But My people do not know The law of the LORD. Jeremiah 8:7
Disasters often follow Disobedience
If we take a closer look at Bible history, almost every disaster recorded was the result of someone violating the Law of God.
Jesus told us that God's original intention was for each man to marry only one woman when He created Mankind (Matthew 19:8). King David violated this rule by marrying many wives and concubines, and it eventually led to incest among his children, and they ended up killing each other.
Many problems caused by failure to observe God's Laws are still happening today. For example, when God created animals, He gave them green plants as food. Men violate this rule by feeding them all kinds of feeds which have been produced and manipulated in the laboratories. As a result, animals develop different kinds of diseases and health issues.
There are numerous similar examples everywhere in the world. As it is written in the Book of Isaiah:
We are all like sheep that have gone astray; each of us has gone his own way. Isaiah 53:6
Success of King David
King David loves the Law of God and he was called "a man after God's heart" (1 Samuel 13:14). When he followed the Law of God, he was very successful and prosperous. He said in Psalms 119:
Oh, how I love Your law! I meditate on it all day long. Your commands are always with me and make me wiser than my enemies. I have more insight than all my teachers, for I meditate on Your statutes. I have more understanding than the elders, for I obey Your precepts. Psalms 119:97-100
It was the Law of God which gave David the wisdom to be a king.