One of the 10 commandments on the tablets that Moses brought down from the Mount was "You shall have no other gods before me", and the reasoning behind this commandment is clear. The preface "I am the LORD your God who brought you out of the land of Egypt, out of the house of bondage." explains that god alone was responsible for saving his people and leading them from the hostile land of Egypt. God cared enough for his people to deliver them safely from bondage and to force the Pharaoh to let his people go. As a result god is the only one who should be worshiped and held in reverence. When idols are created from materials and then worshiped this is an insult to god and all that he has done for the human race.
God expects to be the first priority because he protected the people of Israel and helped them when they left Egypt, saving them from a horrible existence and being at the mercy of the Pharaoh. When Moses demanded that his people be allowed to leave Egypt and the Pharaoh refused god let his wrath rain down on the land. God also allowed Moses to part the Red Sea so that his people could escape safely, closing the water again so that the armies of the Pharaoh could not follow. When people started worshiping false gods and idols made of gold and other precious materials god let them feel his displeasure over their ingratitude and lack of respect.