Start with the policy of salutary neglect, which was a sort of unspoken agreement between the British and the American colonies that essentially gave colonists the freedom of de facto self-government and control over their bustling trade economy. Until the British nearly went broke after fighting the Seven Years War (1756-1763) against the great European powers on the North American continent, this had been the tacit deal between mother country and colonies.
After the SYW, the British were saddled with enormous debt. They had won the war, but broken the piggy bank doing it. Despite the longstanding political success of the salutary neglect policy, the British felt desperate to recoup some of the costs of their expensive war. They felt justified in passing some of the bill on to the colonists, as they felt that they had done their mother country duty by "protecting" the colonists and ensuring the expansion of British colonialism and commerce on the continent.
At this point, however, the colonists were of no mind to give up the lax relationship they had until that point enjoyed with Britain, particularly when it came to...you guessed it...money. In a series of not-altogether-unreasonable but widely-derided new policies regarding tax policy with the colonists, Britain fumbled the most lucrative corner of their worldwide empire with overbearing new policies and in the wake of radical flashpoints like the highly-sensationalized Boston Massacre. Tensions rose rapidly, and it was only a matter of time before diplomatic relations crumbled.
There are too many aggressions to list, and too much history to choose a definitive starting point, but the consensus among most U.S. History scholars these days is that the liminal moment was finally reached in 1774 with the passing of the Boston Port Act. The Boston Port Act is the first of five Big Bad Policies you need to know about. Known as the Intolerable Acts, the series of attempted policy changes was initiated in 1774 after colonists posed several successful boycotts and still-small groups of radical separationists planned dramatic protests like the Boston Tea Party.
Boston Port Act: shut down the port of Boston in a punitive measure directly related to the BTP. As the center of the patriot movement, Boston was arguably the most radical city in the colonies, in regards to their zeal for separation from Britain. However, shutting down the port of Boston was a fairly harsh move, considering that the port served an enormous region, and radical patriots were still just a tiny, if vocal, minority.
Massachusetts Government Act: wrested political control from local colonist-organized systems of government in Massachusetts, and reinstated power in local governors, who were of course appointed by the British. This fundamentally changed the shape of colonial politics. Local outrage crystallized quickly, and it soon spread out in the form of rampant paranoia in the other colonies. It was no longer just radical groups who distrusted the British; now, regular citizens began to question the fairness of the colonial system, and they feared for their rights and their safety.
Administration of Justice Act: allowed for British officials accused of crimes to be tried in England rather than in the colonies. This was ostensibly enacted to protect the right of British officials to a fair trial, but it opened the door for corruption and impunity, as officials were less likely to keep themselves accountable under these circumstances. Furthermore, the logic behind the act had one major historical event to undermine it: After British soldiers opened fire on a crowd of colonists back in 1770, killing five, the future United States President John Adams represented the British defendants in court, arguing that it would set a terrible precedent to drop the principle of fair trials for all. To many Americans, the AJA showed a blatant disregard for the legal and political system the colonists had built.
Quartering Act: required that colonists provide shelter for British troops under any and all circumstances. The original idea was that the colonists should be responsible for building barracks, as the British believed that their soldiers were the main protectors of the Americans. If barracks could not be built, or were full, Americans were required under this act to open their doors to the British. It only seemed right that Americans show some hospitality. For Americans, however, this was an egregious attack on American privacy, safety, and self-determination.
Quebec Act: expanded British territory south from Canada and established a Catholic mini-state in formerly French territories in Canada (e.g. the well-populated Quebec). Seeing as many of the first European settlers on the continent (including English Protestants, who eventually formed the core of colonial society and government) left their respective countries to escape the reach of the Catholic Church, this was an unwelcome development. Although freedom of religion was one of the main tenets of the early "dream" of America, this seems to have only extended to Protestant Christianity.
After the Intolerable Acts, the First Continental Congress was called to discuss the best colonial response. By the end of 1775, war had broken out between the colonies and the British. In 1776, the colonies officially declared independence.