I did not respond to this question because I think it is far too complex for a single paragraph or discussion thread, but the issues between the North and South were longstanding. Certainly, the Deep South considered the threat to slavery as a sufficient justification for secession, but they mentioned other issues as the conflict intensified and these other issues had been mentioned before as potential threats to the Union, i.e. economics, cultural differences, etc. The North, not the South, advocated secession as early as 1794 when it seemed they were going to lose control of the government. The issue of slavery in 1794 was no where near as important as it was in 1860, yet the North threatened to leave not only in 1794, but in 1803 and 1815. And even before the Constitution was written and ratified, as well as during the Phil. Convention of 1787, everyone knew the North and South were different, not just because of slavery, but because of climate, geography, culture, and economics. There was a threat to the Union at that point and armed conflict was mentioned as a possible outcome of the dissolution of the Union.
The underlying issue is did the South develop the way it did because of slavery or was slavery developed because of Southern culture. I think it is the latter. Slavery was the millstone of Southern society, a means to maintain an economy and a social order, and by the way a social order Northerners also readily accepted once black Americans were introduced to many Northern States. Southerners often exposed Northern hypocrisy on the issue of race.
Overall, the question of why slavery should be asked, not if slavery. Was it moral, economic, political, or philosophical? The political implications far outweigh the other issues, meaning the North and South were locking horns over who would control the central government and its spoils, i.e. Western land and the treasury, and what type of political system would reign supreme, a federal republic or a national government. Southerners argued that the Confederate Constitution was a return to the true intent of the U.S. Constitution as ratified.
As for other issues, read this report from the Confederate Committee on Foreign Affairs written by Robert B. Rhett: http://www.abbevilleinstitute.org/review/report-of-the-committee-on-foreign-affairs/