Slave owners used slaves to harvest crops and other resources like rice or cotton which they sold for a living and the abolishment of slavery was a problem for people whose livelihoods depended on it. 

Slavery was unethical and abolitionists like Harriet Tubman and Fredrick Douglas, who were freed slaves,  fought to end it. The opposite can be said for people on the other side who saw nothing wrong with it and fought to keep it. 

The government tried making compromises to satisfy both parties like the Missouri compromise of 1820 where Missouri would become a free state but Maine would become a slave state or the Compromise of 1850 where California would become a free state but there would be stricter fugitive slave laws that prohibited them from becoming freed slaves. Despite compromises being made, neither were happy and the Civil War started after Confederates attacked Fort Sumter.