The Declaration of Independence famously states that
"Governments are instituted among Men, deriving their just powers from the consent of the governed."
Today, this doesn't seem a particularly radical idea. Indeed, most of us take it for granted. But at the time this was revolutionary. In examining just why this was the case, we need to remember that democracy as we understand it today didn't really exist in 1776. Governments the world over did not derive their powers from the consent of those they governed. Monarchies, for example, derived their powers from custom and tradition. In some cases, monarchs even claimed to rule by divine right, exercising powers given to them directly by God.
The experience of more traditional forms of government had clearly not guaranteed the American colonists what they believed to be their inalienable rights. It's not surprising, then, that the Founding Fathers were so keen to establish a form of government of which the people would ultimately control.