The main reason was that so they get to have the land.
They empowered the federal government to prevent voting rights discrimination at the state level; it also resulted in an increased number of minority voters and black participation in the political process.
The Roman culture benefited the Greek stoicism
Manifest destiny is the belief that America should own land from the Atlantic to the Pacific Ocean. People wanted their freedoms and rights to be distributed across (horizontally) the whole continent. It was a form of pride for their country.
I hope this helps!
~kaikers
