Social pressures during colonization

Many colonists belonged to families that had been negatively affected by the enclosure of common lands in England. Their poverty acted as a push factor towards North America. In contrast, very few members of the aristocracy settled in the colonies. In addition, slavery was a major avenue of (forced) migration to North America, and its main drive was also economic.

On the other hand, many colonists belonged to Christian minority groups such as Puritans and Quakers. The Reformation opened an era of intense religious persecution and fighting in Europe. Greater religious freedom was possible in North America because the Church of England did not enjoy official status in the colonies (as it did in Britain). The voyage of the Mayflower is a well-known case of religiously-motivated migration.

There were other factors too, but I would argue that these two were by far the most important.