There were several reasons actually. First of all, there was a war before. As it is well known, wars are very expensive. Soldiers needed to be equipped, soldiers need to be paid, etc. Then came the need to pay for heavy costs. Who to tax? For some reason the Colonies were chosen, perhaps as the war was quite literally fought by the colonies.
The taxes was set to replenish the funds of British parliament after the Seven Years War. The British claimed that it was an expense the colonies had to pay as the British were fighting for them.
