England had just beaten France in the Seven Years' War in America. Doing so cost money, and England was in debt. Even more than that, now that they had taken over more land, they had to pay to keep it secured. They didn't want the French to come and take the land back. They would need to station their British soldiers at forts all over the country, which would cost even more money.
King George and his advisors thought that putting a tax on certain goods that the American colonists used would help pay for the war. King George felt that he had helped the colonists by getting rid of the French and that they should have to help pay for the war.