Manifest Destiny, the notion the U.S. has a moral and religious obligation to spread throughout the American continent, is rooted in the twin notions of Romantic nationalism and American exceptionalism. Romantic nationalism constructs a vision of U.S. political and social life that is morally, religiously and politically ideal. According to this version of nationalism, the U.S. represents objectively the best way to organize political and social life. Thus, U.S. citizens have an obligation to "help" other nations achieve this advanced status.
American exceptionalism suggests that U.S. expansion will not include the negative imperial effects of the expansion of historical empires such as the Roman and Spanish empires. It claims for U.S. citizens a kind of enlightened status which ensures that expansion delivers moral and political goods such as liberal democracy and capitalism, rather than imperial control and oppression.
pilgrims knew that there was more land to be conqurered they wanted to grow intot a world power. they knew there was land westward and that some of the land no one conquered it yet so they wanted it, to have a 'whole' country