Why was the American Civil war happen?

People say the main reason was slavery but there were other reason why. Both the North and the South had big Compromises on states if they won't them to be a free state or a slave state. Also they have a lot of States’ Rights changed in between. What anger the south was The Election Of Abraham Lincoln, because they knew he was going to change the South. So after the election, The south started to secede from the US which anger the North. But the war didn't start until the south shot fires on Fort Sumter which started up the war.