Follow the money. The north was just fine about letting the south go its own way until they started losing a lot money to southern ports. Most people in the north didn’t give a rat’s furry behind about slavery. I’m not saying slavery is right. The institution had to go, but before we get all self righteous about ending it with a destructive, nasty civil war, consider that it would have eventually gone the way of the dinosaurs and lamp lighters as production became more automated.
How were they losing money to southern ports?
...it would have eventually gone the way of the dinosaurs and lamp lighters as production became more automated.
The first commercially viable cotton harvester wasn't introduced until the 1930's. Pesticides and fertilizers came later. Would slavery till then be OK with you?
This isn't quite true. Most in the North hated slavery, but not for the reasons we have been led to believe.
The main reason Northerners hated slavery is because most of them earned wages by trading their work for money, and the thought of someone coming in and working without being paid was a serious threat to their livelihood, and of course people are very passionate about threats to their income.
The second main reason Northerners hated slavery is because they hated black people. We know this because we can read the laws they passed in that era where they express great hatred for the idea of blacks living in their states. The Illinois black codes (The Land of Lincoln) are horrible. They passed laws which would do horrible things to black people if they tried to live in Illinois, up to and including selling them into slavery.
So yeah, the majority of the Northerners were not motivated by the milk of human kindness. They were motivated by hatred and by concern for their own financial welfare.
Their reasons were really ugly, but they don't teach that in history.