The colonists were upset because this was the first time Britain had really bothered them. Up until that point, Britain had left the colonies alone, but when the Stamp Act was put in place the colonists realized that Britain thought they were above the colonies. Colonists didn't like that Britain was forcing them to pay, either. The Stamp Act put a tax on all printed things. Colonists were also angry because they weren't represented in Parliament (British Government) and one of their beliefs was in representation.
The colonists were upset about the Stamp Act because it imposed direct taxes without their representation in Parliament, created an economic burden, and represented increased British control. Protest groups formed in response, emphasizing the principle of "no taxation without representation." This opposition contributed significantly to tensions leading up to the American Revolution.
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