Why did people in the north and south have different opinions on slavery?

People in the North and South of the US had different opinions on slavery because most people in the North saw slavery as both a moral evil and a useless institution, since slave labor was very minimal in the northern states. People in the South, however, felt that slavery was an integral part of their well-being and economy, since their economy was almost entirely agricultural, with a large number of slaves being worked in the fields. 

Spanish conquistadors demanded labour from natives.