What were the major factors that caused the American Revolution?
Although there were a substantial number of grievances by the colonists against the British and particularly Parliament, the primary cause was that the colonists believed they had been denied their "Rights as Englishmen."
Prior to the French and Indian Wars (there were actually four of them) the British government had been preoccupied with domestic problems and the colonies were largely ignored. After the Treaty of Paris of 1763, however, the then Treasury minister, George Grenville, was determined to bring the colonies into line.
Grenville's first decision was to keep British troops in America, ostensibly to prevent attacks from hostile Indians; but in reality because a number of British officers were politically influential and he did not wish to have to deal with them. Keeping them in America kept them out of his way.
Grenville also decided that the colonies must bear some cost of the war. He proposed to do this by imposing a tax on legal papers, playing cards, etc., the famous "Stamp Tax." Colonists had paid taxes previously; however this was the first attempt to tax the colonies directly for revenue purposes. It caused violent dissent, so much so that the tax was never collected and the Stamp Act was repealed. However it had set events in motion and a somewhat vociferous disagreement over Parliament's right to legislate for the colonies erupted. The colonists maintained they should only be taxed by their duly elected representatives, as were all Englishmen. Since they had no representatives in Parliament (nor did they want them) to tax them was to deny them their rights as Englishmen. Parliament retorted that since the colonies were part of the British Empire, they were represented in Parliament, since it represented all the Empire, a theory known as "virtual representation."
Their were other instances which exacerbated an already difficult relationship, the Coercive Acts, the Quartering Acts, etc.; but none of these were the sine qua non of the Revolution.
The colonists position was perhaps best expressed in the Olive Branch petition forwarded to George III (and ignored by him) after fighting had commenced:
The apprehension of being degraded into a state of servitude from the preeminent rank of English freemen, while our minds retain the strongest love of liberty, and clearly foresee the miseries preparing for us and our posterity, excites emotions in our breasts which, though we can not describe, we should not wish to conceal. Feeling as men, and thinking as subjects, in the manner we do, silence would be disloyalty. By giving this faithful information, we do all in our power to promote the great objects of your royal cares, the tranquility of your government and the welfare of your people.
We ask but for peace, liberty, and safety. We wish not a diminution of the prerogative, nor do we solicit the grant of any new right in our favor. Your royal authority over us, and our connection with Great Britain, we shall always carefully and zealously endeavor to support and maintain.