I don't get my history from Wikipedia, but it's easy to link to and lays it out pretty plainly.
http://en.wikipedia.org/wiki/Missouri_Compromise
Although a good high school history book could give you the same info.
The economic system of slavery was based on land. For that system to survive, it needed more and more land. And to maintain the slaveholders political power, they needed to make sure that free states did not get the upper hand in the Senate. The slave states already had the upper hand in the House due to slaves being counted at 3/5 person for purposes of assigning representatives.
Both the Fugitive Slave Act and the Dred Scott decision meant that *no* state could outlaw slavery within its borders. So much for the slaveholders much vaunted 'states rights.' They didn't care about rights, only about their own property.
And to George Purvis, who asked 'why shouldn't property be returned?' - well, that was the South's argument, forgetting that they weren't 'property' under the laws of the states they were *in*. Again, so much for 'states rights'. The slave states were for 'states rights' when it was *their* rights, not so much when it was other states' rights. By your argument, the slaveholders should have had the right to retrieve their 'lost property' from Canada and other countries as well. If you're going to argue for State Sovereignty, then you can't in good conscience defend the FSL.
I don't know what more information you need - the info on the MO Compromise and the Fugitive Slave Law are readily available in lots of places. It's basic history.