Answer:
The material justifications behind Manifest Destiny, are, in first place, the need for more land, because the United States was growing fast, both due to natural growth, and also due to immigration from Europe.
The second material justification was the need for more resources. The American West has important resources, including access to the Pacific Ocean.
Ideologically, the first justification is that Americans considered their duty and right to expand the American ideology over the western part of the continent, and impose this ideoloy over Native Americans.
The second ideological justification is the same but pertaining to religion: Americans felt that it was their duty and right to spread christianity.
