There are two primary reasons which are reasonably related. First, after fighting the French and Indian Wars and defending their homeland, the colonists began to see themselves more as Americans and less as citizens of Great Britain. This evolution of a separate identity made the colonists less tolerant of perceived abuses by Parliament and the British soldiers still stationed in the colonies. It thus appeared to the colonists that the time had come to effect a separation. Thomas Paine considered this outcome as inevitable:
I HAVE never met with a man, either in England or America, who hath not confessed his opinion, that a separation between the countries would take place one time or other: And there is no instance in which we have shown less judgment, than in endeavoring to describe, what we call, the ripeness or fitness of the continent for independence.
As all men allow the measure, and vary only in their opinion of the time, let us, in order to remove mistakes, take a general survey of things, and endeavor if possible to find out the VERY time. But I need not go far, the inquiry ceases at once, for the TIME HATH FOUND US. The general concurrence, the glorious union of all things, proves the fact.
A second reason, seemingly inconsistent with the first, is that the colonists believed that they had been denied their rights as Englishmen, more particularly the right to be taxed by their own elected representatives. This was in response to Parliament's attempts to tax the colonies by way of the Stamp Act in order to pay the costs of the wars noted above. Thomas Jefferson carried it a step further and said that the colonists had been denied the "unalienable" rights inherent to all people; therefore they people of the colonies had the right to change their government and in essence become free and independent.
It was arguably unlikely that the second reason, denial of the rights of Englishmen, would have carried so much weight were it not for the underlying belief that the time had come for independence.
The american revoloution started due to three events, yet more are just precursors:
1) British government retaliating and starting conflict.
2) Taxation of goods and property.
3) persecuting the religious communites and free liberties.