I'm just adding on to the answer above-the North and South had disagreed on multiple political issues since the end of the Revolutionary War, one of which being slavery. (While the attempt to abolish slavery by the North (or the Union) was part of the reason for the formation of the Confederacy, there were others.) Although, it can be assumed that Lincoln being elected into office was probably a main reason for the South's secession , because he wanted to abolish slavery.
