Why did Southern states secede?

They believed they could be more economically prosperous without the North.
They believed the Union no longer represented their interest.
The Supreme Court decided they were legally able to do so.
They wanted to re-align themselves with England.

The first two reasons are the most commonly cited motivations for Southern states seceding from the Union in the lead-up to the American Civil War. The Southern states felt that the North was imposing unfair economic policies that would harm their livelihoods, and they also felt that the federal government no longer represented their interests and values. The idea of re-aligning with England was not a primary motivation for secession.