What famous document begins: when in the course of human

The Declaration of Independence of the United States

If your asking the reason why they did this is if you look at the process at which they did it. The articles of confederation failed as they gave government few powers. So in a basic social contract theory, they gave away a few rights and or freedoms for a mutual benefit. Also written in the constitution there is checks and balances, which was brought up in the federalist papers. This gained support for a new and updated government.