Northerners were concerned that slave owners were going to spread slavery into the territories and even into the Northern states (Dred Scot decision). They thought they were fighting for the country as it was.
Northerners for the most were indifferent until they were told the cotton they once got from the south was going to Britain and France. A lot of jobs in New England and New Jersey would be lost. They were also told the goods they manufactured would not compete well in the south if the tariffs were reduced. That threatened foundries, machine shops, textile mills and many other “protected” industries. The south was equivalent to a colony for the north: a source of cheap raw materials and a captive market for high-priced goods.
Slavery was very much confined to the south and was a dying institution. Most countries outlawed slavery without a civil war and many did not act until after our civil war.