It is easy to say what Manifest Destiny is, but somewhat harder to know for sure how it came about. It is not always possible to know for certain how or why certain ideas come to be popular.
Manifest Destiny was the idea that the United States was destined by God to spread out and occupy a large portion of the North American continent. There was no exact amount of the continent that the idea said the US should occupy. The idea was simply that the US was destined to expand. The reasoning behind this idea is very important. Believers in Manifest Destiny held that the US had a superior system of government, a superior religion, and superior racial makeup.
So how did this idea come about? We can speculate that it came about because Americans were more successful and modern than the people they came in contact with. They were clearly more modern and powerful than the Native Americans and therefore felt superior to them. They were arguably more energetic than the Mexicans and they definitely had a more stable system of government. In addition, the US was the world’s only real democracy at the time and they felt that that made them superior to Mexico and to Great Britain.
Thus, it is easy to say what Manifest Destiny was, but harder to know for sure why people believed in that idea.