The short, trite answer is: States Rights.
Now, to understand that, one really has to understand something America's First War for Independence and the governmental form our Founding Fathers set up. (Stonewall Jackson called the War Between the States [hereafter referred to as the WBtS], "Our Second War for Independence".)
I'll try to be brief in my set up: If I recall correctly (it's been several years since I studied this), during the 1770-80's, the US was bound together with the Articles of Confederacy (which I don't have a copy of at my finger tips). In 1789, with the ratification of the Constitution and the first 10 Amendments, the Federal government assumed more power (necessary to hold the 13 Colonies together as a unified nation), yet the States were still considered sovereign. This is where it really gets important.
I have in the past read some of the original and revised state constitutions--but it's been so long ago that I can't remember anything much about them. However, I do know that post-WBtS, many of the constitutions were changed to reduce the power/sovereignty of the State.
It was perfectly legitimate for the Confederate States to secede from the Union as it (the Union) had ceased being beneficial to the Southern States. They were in essence doing the exact same thing that their forefathers had done roughly 90 years before. The following lines could be applied to the CSA: "Prudence, indeed, will dictate that
Governments long established should not be changed for light and transient causes; and accordingly all experience hath shown, that mankind are more disposed to suffer, while evils are sufferable, than to right themselves by abolishing the forms to which they are accustomed. But when a long train of abuses and usurpations, pursuing invariably the same Object evinces a design to reduce them under absolute Despotism, it is their right, it is their duty, to throw off such Government, and to provide new Guards for their future security." (Declaration of Independence)
The WBtS was long in coming. It didn't just happen overnight. I remember reading
something that James Madison said back in the 1780's or 90's that basically predicted the split between North and South. The problem was political, as well as religious. I am clearer on the politics of the issue than I am the religious differences, but I still want to bring them up. The South was more of a Scotch-Irish Presbyterian stripe (that does not mean everyone was Presbyterian of course! Lee was Episcopal, my own g-g-g-gandfather was a Methodist...) and the North had become more Unitarian/Transcendental (which 'theology' is still rather vague in my mind).
What about slavery? This is where the politics come into play. We always hear and the coloration of so many books and movies portray that slavery was the ultimate issue. Well, it wasn't. It certainly was a factor, but at the start is wasn't so much about "Let's free the slaves!" as it was "Let's keep any more territories from coming into the Union as 'Slave States'." Later, (about 1862 or '63 if I remember correctly), Lincoln did use it as a
so-called 'moral edge'--but I really think he could have cared less about the slaves. (The Emancipation Proclamation only 'freed' slaves in the areas of the south under Confederate jurisdiction.)
To go back to the Free State/Slave State issue...it wasn't about the morality or immorality of slavery so much as it was about the balance of political power. The Southern "Slave States" wanted Missouri and Kansas to come in as 'Slave States' because then there would have been (more likely), like minded men in the political realm. You
see, the Yankee states (including the Mid-West in there) had a greater number of people; therefore, they had more districts and thereby, more representatives in Congress--who naturally wanted to please their constituents. The North was more industrial and the
South more agrarian. Right there, you have a conflict in business (and therefore political) interests.
To bring a practical example of that--the South was required by a Yankee controlled Congress to ship all their cotton through northern ports. They were not allowed to send
it direct to Europe. When the raw cotton (or whatever) passed north over the Mason-Dixon line, the Southron's had to pay a tax. If any of it came back in a finished form (there were very few mills in the South), they had to pay yet another tax. I think, if my
memory serves me right, that that could total up to 75% of the profit that they made from the cotton--leaving them with a measly 25% profit on their hard work.
So, the causes of the war were political, business related, as well as moral. The South was going to leave peaceably (and in my opinion, had Lincoln and his Congress let them, the two factions would eventually have come back together). Some states (such as Virginia) might never had seceded, but when Lincoln called for troops to squelch "the Rebellion"--thus violating the rights of the seceding states--they took their stand for the rights of those states to determine their own destiny. Likewise, he obstructed the rights of other states (Kentucky for one) to make a decision one way or the other, by moving troops in and taking over the legislature before they could vote and setting up a pro-union state government. (I believe that this was also done as an executive order--in other words, without the sanction of the Congress.)
One note I'd like to mention about the so-called "Abolitionists"...these people who are held up as heroes of the highest moral character in opposition to those 'evil slave-holders' (like Stonewall Jackson!)...in many cases were actually Marxist revolutionaries.
Also, some of those 'evil slave-holders', really weren't the keenest on slavery and were
glad that it was actually diminishing. It is my opinion that many men would have freed their slaves, but it would simply have been an economic disaster because when a man freed a slave he had to provide him with "40 acres and a mule"--in other words, subsistence. The slaveholders could not afford to free their slaves--leastwise, not very fast--without completely destroying the South's (as a whole) economy.
So...why do I think the South was right? "We are a band of brothers, native to the soil, fighting for our liberty, with treasure, blood, and toil..." (Bonnie Blue Flag--italics mine.)
With all that said--I live in 2013. When you get down to the nitty-gritty of everyday life, I am an American. A very proud American, who is passionately proud of her ancestry and heritage in the Confederate States of America. I know that the memory of that glorious cause is not a reason to start a second War Between the States--there are honestly more
important things to go at someone's throat for in these days.