This has never made sense to me, it seems like when Lincoln was campaigning he kept saying that he was not interested in ending the institution of slavery he just wanted to keep the union together so what was the big hoopla?
Check the recent AskHistorians threads on the Civil War/secession from the past week. I recall seeing one (though I can't remember which) in which a poster said that the reason had to do with representation in the government, that with the population increasing in the North faster than the South the southern states felt they needed for slavery to expand in order to maintain political balance against the North. As such, they saw the prevention of slavery's expansion to be as existential a threat to the institution as a direct attempt to end it.
If I stumble across the link I will post it.