There were lots of causes. Colonists were upset at certain taxation policies imposed on them by the British, and by the fact that they didn't have representatives in Britain's Parliament to fight against those taxes. Many colonists were also upset by rules imposed by the king that prevented them from settling certain areas in the west. Colonists also really hated having to host British troops in their homes (known as "quartering") which was often expensive and disruptive.
One of the most immediate causes of the war was the Stamp Act of 1765, which was imposed on the colonies to pay for expenses related to the French and Indian War, which was fought in North American a few years earlier. The Stamp Act was so widely disliked that it gave an opportunity for the colonists to come together on one issue; before that time, they had often been divided about their own different interests.
Several other acts, called the Intolerable Acts, were then passed in 1774 and 1775. They imposed limits on how colonists could govern themselves. Colonists strongly disliked having less of a say in their political affairs, which also helped them to bind together against the British.
