It had been building for some time when 1775 rolled around and the first shots were fired. Let's look at a few of the reasons I think were most important:
1) Cultural differences - most of the Americans had never seen England, and never would. They were born here, amidst various religions and ethnicities, and mixed over time to form a new culture. The British, on the other hand, tended to look down at the Colonials, as they called them.
2) Taxation without Representation - In order to pay for the French and Indian War, (to the tune of $15 million) Britain had to tax the colonies, pure and simple. But the colonies had no say in their own taxation, and Britain would only promise "virtual representation" saying that they would look out for the colonies therefore they didn't need members of Parliament.
3) Violence against Americans - The Boston Massacre was essentially a riot where a few British soldiers got nervous and fired on the crowd, but that's not how much of the country saw it. British subjects had been gunned down in a British colony by the King's Army. By the time of this event in 1770, eventual rebellion may have been inevitable.
4) The Sons of Liberty - A group that wanted indepedence for a long time, and used propaganda, newspapers and revolutionary acts like the Tea Party to try and get other Americans to join in a revolutionary effort.
5) British stubbornness and vindictiveness - by overreacting to the Tea Party and closing the Port of Boston to trade, the King united the northeastern colonies in sympathy with Massachusetts, and spread revolutionary sentiment. Then when colonists in Massachusetts sent the Olive Branch Petition after Lexington and Concord to try and repair the damaged relationship, they were turned down flat. Britain threw gas on the fire.