Why did the colonists believe that "life, liberty and the pursuit of happiness" were inalienable rights?

They wanted to have ground rules and assure the people that the freedom they fought so hard for was going to stay that way!

The American Colonists had been granted the basic human rights of Englishmen with the original charters. The Colonists had enjoyed these rights and a great deal of freedom to make their own laws before the French and Indian Wars. This was the historical precedent.

After the French and Indian Wars King George of England revoked these rights on the basis that the American colonists were tenants living in land that belonged to the crown. King George dissolved the local legislatures, deposed colonial governors and installed his own appointees as rulers of the colonies.

John Locke of England had written that no where in the Bible did the King have the divine right to rule. Instead natural law indicated that individuals had basic human rights that did not come from the King or the Government. Locke's writing established that people had basic human rights based on Biblical principles and natural law.

Thomas Jefferson echoed this belief in the Declaration of Independence. Jefferson wrote that because people were created by God they had basic human rights to life liberty and the pursuit of happiness.

Based on historical precedent , Biblical principles and natural law the colonists believed that their rights were inalienable. As these rights did not come from the King or the government, these rights could not be arbitrarily taken away by the King or the government.