Expansionist justified the idea of Manifest Destiny because they said they were spreading democracy. They wanted everybody to be live like they do and to be democratic and what better way to do that than by spreading the word to the people living on the land that they already conquered.
People who must suffer with patience to hide the fear, the terror they have. The white man is obliged to colonize and civilize other races in benefits of themselves. Seek the gain of others and work for the benefit of others. It also expresses to test the pride that is displayed.