I wrote this comment not too long ago. There is also a section about it in this sub's FAQ.

The TL;DR is: the war was caused by slavery. There were a specific set of slavery-related issues that caused it—notably, the future of the expansion of slavery westward, but also the enforcement of the Fugitive Slave Act in the Northern free states, the future of slavery in Washington DC, as well as some other slavery-related issues along those same lines. But every issue that caused the war had to do with slavery.

If you have more particular questions that are not addressed in any of those previous answers, and you would like further information, feel free to ask.