Before I begin to answer this, I would also recommend a look into this thread started by /u/Lorgramoth. While not identical to your question, many of the answers touch upon your very question.

Now the quick and easy answer: there are a number of answers to your question and it is among the most contested questions in the history of early America. I'm going to go through a few of the most popular/debated answers in order to give you an idea of how this question has been answered. Before I begin, however, it is important to note that when armed conflict broke out at Lexington and Concord in 1775, it was far from clear that the war would culminate in an independence movement. The vast majority of the British world, including most American colonists, saw the outbreak of hostilities as New England as a local phenomena. However, the British response and a series of events, including Paine's writings and Lord Dunmore's Proclamation promising freedom to slaves in Virginia that escaped to British lines, pushed colonists towards favoring independence. I bring this up because it is important to recognize that when fighting began, it was not necessarily predetermined that the goal was independence.

Charles Beard An Economic Interpretation of the Constitution of the United States (1913)- Beard is most interested in the creation of the Constitution to address the problems of the Articles of Confederation that first governed the independent United States. However, he also address what the American Revolution was about. Beard argues that the wealthy elite, men like Washington, Jefferson, Hamilton, etc., supported independence (and eventually a Constitution) that would protect their business interests. Beard argues that the leading men of America recognized that following increased British interest in North America after the expulsion of the French in 1763, new measures restricted their pursuits of wealth including ventures in trade and land speculation. In this sense, these men created civil unrest and led a war against the British to protect their own wealth and expand future possibilities. Beard also argues that a protection of their own wealth and fortune was the main drive for the Constitution and a curtailing of state power in favor of federal power.

Bernard Bailyn The Ideological Origins of the American Revolution (1967)- Bailyn analyses dozens of political pamphlets produced prior to and during the American Revolution to understand the rhetoric of American Revolutionaries. In these pamphlets Bailyn recognizes a number of trends; however, perhaps most importantly, he argues that colonists who supported the American Revolution and eventually independence for Great Britain, believed that the British Empire was no longer the a defender of "liberty." These pamphlets placed the British Empire of the late eighteenth century in a longer history individual liberty. In this tradition, the British had long been the defenders of individual liberty and common law where the tyrannical power of monarchs and "Papists" had been held in check by an elected parliament. In this view the British differed from other powers, specifically the French. However, in over the eighteenth century and especially after the Seven Years War, the British government had begun a "deliberate assault of power upon liberty." In this sense the British were no longer defenders of liberty, but rather, “conspirators against liberty." Through unlawful taxation practices, restriction of American trade, and the expansion of British governmental control on colonial government, Parliament had transformed the colonists into "slaves" of the British metropole. Bailyn notes, these discussion were not “mental gymnastics," rather they provided rational grounds for resistance. When Parliament and the Crown reacted harshly to resistance in Boston and crackdown along the eastern seaboard in 1775, American colonists banned together in resistance in order to protect the liberties with Parliament no longer appeared to do.

Gary Nash Urban Crucible (1979)-While Bailyn is obviously arguing an "ideological origin," Gary Nash analyzes the largest American cities prior to the outbreak of war. Focusing on Philadelphia, New York, Boston, Charleston, and Newport, Nash argues that following the conclusion of the Seven Years War, the major port cities experienced widespread economic stagnation and significant unemployment. Sailors and dockworkers who had seen growing employment during the colonial conflicts that existed throughout the early 18th century, were left unemployed and without the prospect for employment following the expulsion of the French from North America. On the docks of the major cities these men began blaming a number of parties for their strife. However, they often turned against the few American merchants and other upperclassmen who were not suffering as much. These merchants, men like Samuel Adams of Boston, John Brown of Providence, and other leaders of the Revolution, were also feeling the effects of British taxation. Looking to exploit the dissent of dockworkers, these men transformed dissatisfaction against themselves into tumult against the British colonial government. By inventing ideas like "No taxation without representation," openly attacking British customs agents and officials, and supporting the creation of militias to defend American interests, these merchants and other upperclass colonists, were able to harness the dissent of unemployed workers, and to a lesser extent farmers, against the British government that was disrupting their fortunes. In this sense, the war began only when the upper echelons of society were able to convince the lower groups to support a war against the British government.

Gordon Wood Radicalism of the American Revolution (1991)- A student of Bailyn, Wood's work is not primarily concerned with why or when the Revolution occurred, but does discuss what it ultimately created and how that process unfolded. In doing so, however, Wood does make a couple of key statements to advance the discussion. Disagreeing with Nash, Wood argues that pre-Revolutionary American colonists were not an oppressed people. In fact, Wood rightly demonstrates that American colonists paid among the lowest taxes in the Empire. Wood agues that Americans were rebelling to overthrow "monarchy"; however, he does not simply mean the king. Rather, Wood argues that American colonists were rebelling to overthrow the idea of "inherited wealth/status." In this sense, American colonists of the late-eighteenth century began recognizing that they could compete with the British in an international market economy and resented that they were subjugated by men who had inherited their position. In this system, even the richest and most privileged men in America would never rise to the status of colonial officials or the hierarchy of the English system and therefore constructed a revolution, and eventually new order, in which they could rise to the top of the social order.

Woody Holton Forced Founders (1999)- moving away from Beard, Bailyn, and Wood's analysis of the elite, Holton focuses on the common man, natives, and enslaved people to address the origins of the Revolution. Holton is also looking at Virginia rather than New England or the major cities. Holton's main argument is that pressure from below forced the upper classes into a revolution. Backwoods settlers opposed the British policy of restricting western settlement after 1763. These same backwoods settlers came into open conflict with the natives in the regions they were settling and found themselves unaided by British forces. Furthermore, these settlers hoped to join the rank and file of the eastern elite and perused the establishment of plantation enterprises based on slave labor. Collectively, these average men were forcing the British to spend large amounts of money restricting their western movement. As the British pushed back against them, these settlers began openly resisting. Stuck between rebellious western settlers and a restrictive British government, the eastern elite, men like Washington and Jefferson, were forced to chose sides and chose the side of the American colonials creating a revolutionary atmosphere.

These works are not the only works explaining the American Revolution; however, they give a good sense of some of the major debates concerning how it began! I'd be happy to suggest more/ give further information if you're interested. Cheers!

Sources:

Charles Beard An Economic Interpretation of the Constitution of the United States (1913)

Bernard Bailyn The Ideological Origins of the American Revolution (1967)

Gary Nash Urban Crucible (1979)

Gordon Wood Radicalism of the American Revolution (1991)

Woody Holton Forced Founders (1999)