In some theories of democracy, the government governs with the consent of the governed. The social contract between the government and the people defines the nature of that relationship, and both parties are forever bound by its terms.
A weakness of this theory is that in most democracies, few, if any, of the "people" have had any chance to explicitly consent to this "agreement". A response to this argument is that each person as he or she comes of age chooses either to consent to the terms of the agreement or leave the country, while those who immigrate indicate their consent by virtue of having immigrated.
Another objection is that such consent ought to be unanimous, and unanimous consent is impractical in a state of any significant size.