What was the main cause of the American Revolution?

Being under the control of Britain eventually became unbearable for the US colonies and they wanted their independence, to control their own destinies.

Specifically, Americans did not tolerate "taxation without representation". The British taxed Americans heavily, but their parliament refused to address American concerns. Britain continued to exercise its military control in support of its mercantile system, but events such as the Boston Massacre and the laws that followed the Boston Tea Party soured relations with the colonies, and armed conflict became inevitable.

(see the related question)