i love the effects of our failed adolescent education systems.
here:
1.) Economic and social differences between the north and the south.
            southern economy became a one crop economy thanks to the cotton gin and the lack of motivation to move towards other crops.  the north was mostly centered around industry, which became mostly centered around the cotton from the south.  classes became established in cities, while the south remained socially indifferent.
2.) States vs. Federal Rights
             some felt (a.k.a. the south) felt as if the newly passed constitution was taking states rights away ignoring their ability to act independently.  some felt that they should be able to participate in deciding whether or not to accept federal order, which brought about nullification.  when nullification was denied by the legislature, states began to talk of secession.
3.) fighting between slave and non-slave proponents
                 when lands acquired through actions such as the Louisiana purchase there were concerns whether or not newly admitted states into the union would be allowed to accept the practice of slavery.  thus bringing the action of the 36.30 line.  after the new lands of the mexican american war were acquired, the wilmot proviso was brought up to ban the new lands, but the compromise of 1850 brought about provisions that would balance between the slave and free states.  things like the fugitive slave act, the kansas- nebraska act (state sovereignty issue, border ruffians, and eventually bleeding kansas and the caning of senator sumner)
4.) the growth of the abolition movement (hopefully no explanation needed)
5.) the election of lincoln
           his interests laid in the "preservation of the union."  slavery was just an external cause that he used to motivate the north to BATTLE.  by the time of his election seven states had already seceded and south carolina declared secession when he was elected in 1860.
there ya go bud.