Why? Admission to the Union is textually committed to Congress by Article IV of the Constitution:
"New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress."
But it doesn't say anything about states leaving the Union; thus, given the tenth amendment, shouldn't it be the right of every state to leave the Union if it so chooses?
In my opinion there isn't a question of whether a state may leave or not. Nothing in the Constitution forbids it. The question is the manner in which they choose to leave. The Southern states seceded unilaterally. When they did so the walked away from financial and treaty obligations leaving the remaing states to shoulder the responsibility for all obligations in full. The took with them literally every piece of federal property they could get their hands on without compensation of any kind. For a brief period of time Mississippi closed the river to all traffic bound for the North, cutting off much of the United States from access to the sea and placing an economic stranglehold on them. So my question is, why do you feel that the Constitution protects only those states leaving and offers none to those remaining? Why can the seceding states use the Constitution as a club to beat the other states up with? Don't the remaining states have any rights at all?
Secession should be allowed, but only with the approval of both sides. Requiring this makes sense because only then are the interests of both sides protected and only then are all matters of potential disagreement ironed out before the partitioning. And I believe this is implied in the Constituiton as well. Approval is needed to join. Once in, approval is needed to partition a state into to or remove a state entirely by combining it with another state. In fact, Congressional approval is needed to for a state to change it's border by a fraction of an inch. Why should leaving be any different?
The Constitution, which upon ratification replaced the older document begins with a preamble which reads:
We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.
The Constitution superceded the old Articles but stressed in its opening that the Union was, is, and will be perpetual.
What the heck don't you Southrons understand about that?