In the beginning...
While you may have heard in Law School (or from the stupid talking heads on television) that the first legal document of the United States is the Constitution, that is not a fact. The very text of the Constitution states that it is preceded by one other document; the Declaration of Independence. "[D]one in Convention by the Unanimous Consent of the States present the Seventeenth Day of September in the Year of our Lord one thousand seven hundred and Eighty seven and of the Independence of the United States of America the Twelfth" - Article VII
So the beginning is not the Constitution, but the Declaration. So what does the declaration say about the founders views on secession? Well if the Declaration outlines American Values then we can say that at its heart, the right to secede is an American Value.
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.--That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, --That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness. Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes; and accordingly all experience hath shewn, 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.
As you can simply read from the Declaration, the Founders cite Natural Rights as the basis of their rational for being able to leave the British Empire. As a matter of fact the founders say that it is "their duty" to "throw off such Government..."
I believe that if you told the founders that the price for joining the Union would be that no state could ever leave the Union, that the was the price of admission so to speak, the Union would never have been formed. They saw the Union as a consensual act among free people.
So what has changed?
Well, Lincoln changed everything with the Civil War. While I and the founders may cite natural law and natural rights as the basis for the right of free peoples to come together as they choose and sever there bonds as they choose, Lincoln taught us that there is something with a little more persuasive force than natural law and rights: the right of conquest. Basically, the Union is forever, "indivisible," and that the Federal Government will use whatever force is necessary to preserve that. In a parallel universe, had you shown the Founders what Lincoln would do to preserve the Union nearly 100 years after the Constitution was approved, I can state with 100% faith that they would never have voted to form the Union in the first place.
So was Lincoln wrong? No, I can't say that. He won. It was for the betterment of mankind. The basic conundrum of the United States is this: E Pluribus Unum. In the years after Lincoln's victory the United States has done a delicate balancing act of how much the republic wants to be "many" and how much it wants to be "one." Basically, how strong the central government should be. Examples abound today. Roe vs Wade made the United States much more of a "one" place. The legalization of marijuana and gay marriage are making the several states much more of a "many" place.
Today we have some people who believe in Federalizing everything. In a recent Men's Health there was an article about putting seat belts on school buses. A noble cause I am sure. The author of the article urged the readers not to contact their local school boards, or even their state legislatures, but instead he urged that they should contact their congressional representatives in Washington D.C. Taken to its natural conclusion, why even have independently elected Governors or Legislatures, the states should simply be administrative districts for the federal government. I think most Americans oppose such an idea, they like the weirdness of Vermont, the "Texasness" of Texas and the "nice" of Minnesota.
So we come back to the present. What is a rational way to have a discussion about secession? Firstly we have to admit that one day, either ten years from now or a thousand years from now, a state will eventually leave the union. That day will come, how we react to it will be what matters. If you would have told the average American ten years ago that we were going to have gay marriage, a black president and legal marijuana in a decade, they would have told you that you were nuts. Change happens, sometimes faster than you can anticipate.
Best Case Scenario?
There are many examples of the wrong ways and the right ways to deal with different groups of people who don't want to live together. A sure fire way to a civil war is to make people live together when they despise each other. The United States can look to the break up of Yugoslavia as a lesson in how not to handle secession. There would be no modern Lincoln. Instead you could imagine an American President standing trial for crimes against humanity in Europe for his actions trying to hold the Union together. Instead of coming apart like Yugoslavia the United States should look towards peaceful separations, such as the dissolution of Czechoslovakia. There should be a high bar for a State leaving the Union, I suggest a 2/3rds vote of the people in a plebiscite, maybe 3/5ths on the low end, but a super-majority would be a precondition. We can all look to the founders for guidance on this, the Declaration is a road map for how a proper secession should go. The super-majority requirement would make the requirement that "Prudence, indeed, will dictate that Governments long established should not be changed for light and transient causes;." Which follows with natural law and natural rights. In the end we have to ask ourselves this question as Americans, if for example the people of Texas vote 90% in favor of leaving the Union, is it right to force them to stay in? What sort of nation would that make us? The bar must be set high, but eventually a state will leave the Union, it would serve us well to prepare for it before it happens.