#4 One of the reasons were laws that the South didn't like were passed. The North passed a law that some states could be slave free. The south didn't like that
#3 Economic differences between the North and the South was also a problem. The South had more cotton production but the North had more railroads and ammunition.
#2 As America started to expand, first with the lands from the Louisiana Purchase, there was an argument whether new states added to the US would be slave or free.
#1 When Abe Lincoln was elected in 1860, the South was mad with their new president. They believed that Lincoln was an abolitionist (which he was) and preferred of the North's ways.