What does the preamble have to do with the social contract theory

The social contract theory holds that peoples' moral and political obligations are dependent upon their agreement as a society. It is one of the earliest thought concerning human settlement and cooperation. It is as old as philosophy itself. The Preamble to the US constitution is informed by the social contract theory since it declares and proclaims the obligation of the people of the US in coalescing together to form a more perfect union grounded in their desire for liberty, and fro the purpose of posterity of their agreement for their future generations.