Why were the colonists upset about the Stamp Act?

The colonists were upset about the Stamp Act for three main reasons. The first reason was that the Stamp Act taxed a widely-used resource that was needed by many individuals, especially in the merchant class, and was implemented in a way that prevented significant circumvention through smuggling or other methods. Thus, the colonists were broadly impacted and had few avenues to dodge the tax, which imposed a burden on the growing class of individuals involved in owning businesses.
The second reason was that the tax was viewed as a means of raising revenue for England rather than as a means for regulating trade among and with the colonies. Taxes meant to regulate trade were not popular, but they were more widely accepted because the colonies were a part of England. Further, measures meant to regulate trade were more likely to impact all members of the British Empire, including those in England. Measures to raise revenue, on the other hand, could be targeted at the colonies to avoid constituent anger at home, thus making revenue-raising taxes an increased burden on colonial residents.
The third reason was that the colonies were not adequately represented in Parliament. Tax-raising measures were less popular in England because the representatives did not want to lose electoral support. However, because the colonies were not represented, they had little recourse to make their concerns about new impositions known.