You know, you can look things up on the internet.
As I said earlier, it was all about the money. Pretty much everything is about the money.
Part of the problem began with the Tariff of 1828. You can look it up but, briefly, it greatly benefited the north's cotton mills while costing the south $$$ via a heavy tax on imported goods. That created a mistrust between north/south.
There was also the whole "free state/slave state" thing. As everyone knows the House of Reps is based on population, the Senate is two per state. In the 1850s, the "free states" outnumbered "slave states". The balance of political power had entirely shifted. Some in the south felt that this was going to turn them back into colonies.
As time went on, abolitiionists began calling slavery "immoral" as opposed to "evil". That didn't float with the church-heavy south.
Around this time, there were several skirmishes in the midwest wrt to the whole "free/slave state" thing(John Brown/Bleeding Kansas). More mistrust.
And Lincoln was elected.
To summarize, slavery was a key issue but it was about $$$ and political power. The south had no business model that didn't include slavery and the Tariff of 1828 put a financial strain on their ability to import goods. Essentially forcing them to buy from the north. Which led to mistrust when the political power shifted to the north 20+ years later. We laugh at third world countries having fist fights in Congress(or whatever they have) but that was us in the mid-1800s. The south truly felt they were a different country and that State's Rights were being lost. So they seceded.