Ooo I got this one!
Manifest Destiny goes back to the Doctrine of Discovery. The Doctrine of Discovery was declared in the 15th century by the Pope. This Doctrine said that Christian nations had the right to claim lands that weren't occupied by Christians in the name of their Christian countries. Basically, it was open season on lands that were occupied by indigenous peoples. This brought explorers out all over the place during the Age of Discovery in a massive land grab. Though the occupation of the U.S. Colonies may have not been the reason for colonization of what would become our country, the people still came here, and set up shop, so to speak. The Doctrine of Discovery instilled the idea that lands needed to be claimed in the name of God-given right. This idea of lands being God-given also was perpetuated in the 1800s, but the term Manifest Destiny was coined by a newspaper editor in 1845. The Second Great Awakening also fueled Manifest Destiny in that people began a religious revitalization, further perpetuating the idea that God himself was pushing people towards the American West. Additionally, economic opportunity also pushed our country westward. There were vast spaces of land to claim as much as one wanted. The discovery of gold in the west also made promises of wealth to be gained. I'm sure there are more reasons too, but this is what I know through a combination of quick research, as well as my years of University study. Hope it helps answer your question!