Monarchies and a happy populace never go hand in hand.
A republic promotes the individual, and thus stimulation of the innate intellectual and moral curiosity of individuals, because they are to govern their own lives. Consider the economy of the United States compared to other nations. Our republic has facilitated an economy that (no longer at it's peak due to private sector constrictions) is one of innovation; our populace is wrought with creativity, drive, and personal aspiration that is absent in a monarchy, as such economies are based more of manufacturing.
A single ruler inevitably may lead to corruption, and now individual should blindly follow a leader due to a superficial title. Such idea could realistically lead to regimes such as Nazi Germany or Soviet Russia. No man is stronger than the many minds the constitute a nation. A true leader can only be looked to as a hero is voted in by the people.
Now, the system which republics use is spot on. But Constitutional and Parliamentary monarchies do it better. The monarch provided a person that the people of a nation can look to as a hero. The constitution or parliament can keep this monarch's power in check. The only real problem have with this government is that the king should have more power in the government, but not too much.