Answer: The Fugitive Slave Act required all citizens to assist in the capture of escaped slaves, which the North didn't approve of, as they wanted to end slavery.
Explanation: In many history classes, you'll be taught that the North wanted to free all the slaves and opposed the South's cruel use of slavery, although all states similarly took advantage of those who were slaves.
If the Northern states were to approve of the Fugitive Slave Act, they'd want to support slavery in the South, which wouldn't connect to what you'd be taught in school.