Why did the north so strongly resent the fugitive slave law, and why did the south resent northern resistance to enforcing it?

The fugitive slave law was meant to return slaves, who had escaped to the "free" North, back to the South. It was a compromise set up by previous agreement made most recently in 1850. The 1850 agreement was an attempt to amend slave laws, but still allow for slavery in the South, while allowing the country to further expand to the West. The North resented the law, because it denied slaves the freedom embodied in the North. The South resented it, because they saw it as their right to own slaves per the 1850, regardless of where the slaves were in the U.S. The slaves were deemed extremely necessary because of the requirement of slave labor in the vast agricultural networks in the South.