Now I want to start this by saying I am not a believer in any of the lost cause, Gods and Generals style historical myths. The war was caused for mostly economic reasons by a wealthy plantation owning elite, and poor people fighting for the wealthy elite has happened throughout history. However, it generally comes with some motive shifting propaganda attached to it. With the Philippines and Cuba it was nationalism and a growing pride in the country and a desire to make the country known. In Vietnam it was the paranoia surrounding communism. In Afganistan it was 9/11 and Iraq WMDs.
What was the south’s propaganda excuse for the common farmer? Because to me from the common Southern farmer’s perspective it would be like if Jeff Bezos asked us to fight a war to defend the use of AI instead of manual labor because the government was about to make it illegal. Like slavery was not only a moral sin of the highest caliber, but also allowed these rich plantation owners to buy up a bunch of smaller farmer’s land and became monopolies in their own right. Its the same thing Romans would argue over in the senate. So why fight a war defending the people that are economically bullying you?
More can always be said but this older answer should be of interest.
Listen to Episode 108 of the AskHistorians Podcast an interview of the historian Keri Leigh Merritt by /u/ThucydidesWasAwesome. It might be helpful to you.