The South seceded because of split ideas between the Union and the South. Well, the major causes are considered to be slavery and right of the states. The states in Confederation was issued an “article of Secession” to show the reasons why they would leave the Union, with Texas, Mississippi, Georgia and South Carolina, sending in additional document called the “Declarations of Causes”. The major themes to come out were, you name it, slavery and state rights. All four of them defend slavery claiming it to be related to the rights of the state. Ironic, but well, it is what to expect when rich people don’t want to let go of their free labor. Also back then, the so-called American freedom isn’t really that open minded. Other grievances such as exploitation in of the military and economy are in the documents as well.
To be fair, all four of them declare that defending the right to slavery is their main objective. Georgia and Texas demanded slavery to be expanded, that it should be a right given to states in the Union, stating that it was unconstitutional of the Congress to deny so. The abolitionists (anti-slavery) are attacked in this case as a reason that they are provoking violence to get what they want, that they are using the propaganda of sort to create prejudice. Mississipi and Georgia point out that slavery plays a huge part for the Southern economy that removing them would destroy the economy (which goes against the economic theory, paying the slaves increases consumption and would boost the economy further). In the argument for states’ rights, there is an actual argument that the Union should had enforce the Fugitive Slave Act of 1850 that mandates slaves to be moved to the South. So there you have it, differing ideology, with slavery as the root cause of the secession.