The colonists were mainly upset about taxation because they believed it was an infringement on their rights and liberties. They argued that they were being taxed without their consent and without proper representation in the British Parliament. They felt that they had no say in the decisions made by the British government, and therefore, they should not be required to pay taxes to support that government. This idea of "no taxation without representation" became a rallying cry for the colonists.
Moreover, many colonists were struggling economically, and additional taxes made their situation even worse. The taxes imposed by the British government made it difficult for colonists to make a living and provide for their families. They believed that the taxes were being unfairly levied on them to support British businesses and the British economy.