Why was the stamp act upsetting to the colonists?

Because the stamp act was a tax on all paper goods, they had to be paid for then received a stamp. It was another tax the colonists didn't want or need after governing themselves for so many years after the King had left them alone. They felt that it wasn't his right to tax them or in-force rules because he wasn't in the Americas and it wasn't the way they were used to being governed, plus they had no seat in the government with the King, so their voices went unheard. The stamp act was just one way/thing that angered them on a list of many.