The North was primarily fighting to abolish slavery while keeping the Union in tact. The North was upset when the South officially left the colonies, since it made them look weaker to other countries, as they themselves were on the brink of civil war. They did not want secession to be legal for this reason, but at the same time they did not agree with the South's view on slavery.
The nuclear factor changed it but i think it aslo fought
Great Britain required more taxation from the colonies because of the French and Indian War
Economists have frequently hypothesized that industrialization and its correlates played a major role in inducing fertility decline in the United States after 1850. ... This leaves changes in the cost of raising children as the likely driver of the industrialization result.
