The idea of Manifest Destiny propelled the nation westward because it told Americans that they had a right and even a duty to expand their country.
The idea of Manifest Destiny told Americans that God wanted them to expand as much as possible because America had the best civilization, the best government and the best religion. In these ways, the US was believed to be superior to the Indians and the Mexicans who inhabited the lands that the US eventually took. Since the idea of Manifest Destiny made it clear that God wanted the US to take these lands and make them more civilized and useful, American were inspired to push westward.
Manifest Destiny, then, gave Americans the sense that God wanted them to move west. This belief helped to propel the nation westward.
When John L. O'Sullivan comes up with the wording of "Manifest Destiny," he captured the feeling of a particular moment in time. Manifest Destiny became the belief that America possessed a divine right to expand and to widen its horizons or national boundaries. Manifest Destiny contributed to American exceptionalism, the notion that America was an ordained nation through providence to appropriate anything in accordance to its own subjectivity. This helped to develop progress as something tangible, meaning the acquisition of land and helped to spark a convergence between economic greed and national identity. In both, Americans felt comfortable expanding its boundaries and into any other indigenous people, relegating them to the peripheries as American pride become centrally vital to its expansion. Manifest Destiny became the license to expand "from sea to shining sea."