Why did the North so strongly resent the Fugitive Slave Law and why did the South resent northern resistance to enforcing it?

Because most Northerners were not Abolitionists.

It is true that some disliked the idea of new slave-states on moral grounds.

But most were wanting to hold on to the cotton revenues, without allowing the South to extend their influence in Congress (by the admittance of new slave-states) and reduce the taxes on imports, by which the North was able to protect its manufacturing industry.