How did sectionalism (loyalty to the well-being of one's own region rather than that of the whole country) lead to the start of the Civil War?

Sectionalism was one of the most significant causes of the Civil War. This is due to the fact that the North and South had very different views on the institution of slavery. At this time, many Northern politicians did not want the institution of slavery to spread to the new territories we were acquiring. This is because an increase in slave states would give Southern politicians more power in Congress to make laws that they deem appropriate. The Southerners had the opposite view, as they wanted slavery to exist in all new states the US acquired. This issue would be one of the leading causes of the Civil War.