What caused Manifest Destiny?
Manifest destiny was caused by the fact that the American people were historically connected to English civilization. This gave them a sense of superiority over the other peoples who lived in North America.
Because of the fact that they were white and civilized, the Americans felt superior to the Indians. They felt that the Indians' lack of civilization proved that they were inferior. Therefore, the Americans thought, they deserved to conquer Indian lands.
Because the Americans were English and England was powerful, they felt superior to the Spaniards of Mexico and Cuba and other places. They knew that their country was superior to Spain and they felt that their Protestant religion was superior to Catholicism.
So, because of their origins in white, English civilization, the American people felt they were superior to others and deserved to dominate all of North America.
The social movement of Manifest Destiny that evolved in the 1800s had no one cause, and no one definition. It did not start on a Tuesday and end on a Wednesday. It became, over time, a generally accepted justification for conquering western lands from both Mexico and Native Americans.
I remember a quote from one of my college professors very well. He wrote it on the board and made us prove it or refute it:
Expansion, in the history of Western Civilization, was most often motivated by greed, and justified by religion
I think we can put Manifest Destiny in this category. To take the land and resources that other peoples had owned for hundreds, even thousands of years, a social justification had to emerge. Simple racism or nationalism wasn't enough. We needed God's blessings. While it is difficult to find a passage in the Bible that tells Americans they are blessed to take North America, it was nevertheless easy to believe that we were God's chosen people, and so that was used to justify every conquest, every subjugation in the American west.