Some of the main reasons why it started was when new states were forming, the slave states (kinda forget the name they were called so ima just stick with that) and confederate union had a few arguments deciding which states would be anti-slavery and which are pro-slavery. Which added more tension because the people who where anti slavery wanted everyone to be a freeman as they saw slaves as humans compared to the slave states they saw them as people unworthy to be ‘deemed’ human. Also, the slave states didn’t want to give up slaves because slaves helped them with their lives and it helped them with their income. Another big factor was Abe Lincoln being president at that time. As he wanted to put an end to slavery and the pro-slavery people didn’t agree with that.
For more information about the civil war I highly suggest you to watch over simplified, hes funny and interesting:)