There are multiple reasons for the Civil War but primarily it was the South's realization that they were under the yoke and eventual control of the Northern States. In the South, it was the foreseeable extension of slavery that had to be preserved. In the North, it was the preservation of the United States and the ability of the National government to control commerce to the detriment of some states, in this case, the Southern states.
It was not until much latter in the war that slavery became a central issue for the Northern states, especially in recruitment after the anti-slavery groups in the North had time to educate and make people aware.
Keep in mind, at this time, the average Joe did not travel more than a 100 miles his his lifetime ... most were simply unaware and indifferent.
Lincoln's personal approach to the slave problem was to ship them all to Haiti ... the Ships of Tears much like the Indians by Jackson. That's why most educated minorities have a jaundiced view of Lincoln and hardly consider him a hero.
In Lincoln's own words, on Aug. 22, 1862, President Lincoln wrote a letter to the New York Tribune that included the following passage: “If I could save the Union without freeing any slave, I would do it; and if I could save it by freeing all the slaves, I would do it; and if I could save it by freeing some and leaving others alone, I would also do that. What I do about slavery and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union.”