Why did the southern states believe they had the right to secede of the union?

Southern states believed they had the right to secede from the Union primarily due to the principle of states' rights and the belief that the federal government was overstepping its authority. They argued that the Union was a voluntary agreement among sovereign states, and thus they could choose to leave if they felt their rights were being threatened, particularly regarding slavery. The election of Abraham Lincoln, perceived as a threat to the institution of slavery, further fueled their belief that secession was justified to protect their way of life.