What reason would people give at the end of the 1800s for why the United States should expand outside the continent?

1 reason may be the idea of Manifest Destiny

Another reason may be that the price for some land was cheap, for example when we bought the Louisiana territory for a small price from France

There were also many opportunities for more jobs, and more money

The idea of being a westward person was ideal and somewhat "fantasized"

Also, once they had purchased, the discovery of gold lured many people