Article 4, Section. 3.
Clause 1: 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.
Clause 2: The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.
Article 1, Section. 10.
Clause 1: No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Clause 2: No State shall, without the Consent of the Congress, lay any Imposts or Duties on Imports or Exports, except what may be absolutely necessary for executing it's inspection Laws: and the net Produce of all Duties and Imposts, laid by any State on Imports or Exports, shall be for the Use of the Treasury of the United States; and all such Laws shall be subject to the Revision and Controul of the Congress.
Clause 3: No State shall, without the Consent of Congress, lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.
Art 4, Sec 3, Cl 1: In seceding, a state neither forms a new Sate nor forms or erects a State within the Jurisdiction of any other State.
Art 4, Sec 3, Cl 2: Irrelevant to the issue of secession.
Art 1, Sec 10, Cl 1: In seceding, a state does not enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
Art 1, Sec 10, Cl 2: In seceding, a state does not lay any Imposts or Duties on Imports or Exports.
Art 1, Sec 10, Cl 3: In seceding, a state does not lay any Duty of Tonnage, keep Troops, or Ships of War in time of Peace, enter into any Agreement or Compact with another State, or with a foreign Power, or engage in War.
In seceding unilaterally, a state removes itself from the Union and changes it's status without consent of Congress. Since Congressional approval is necessary for every other change in status then by implication it should be required for leaving as well.
In seceding, a state does not enter into any Treaty, Alliance, or Confederation, etc., etc., etc.
In seceding unilaterally, a state is taking an action which may have a negative impact on the interests of another state. Since Congressional approval is required for other actions by the states which may have a negative impact on the interests of the other states then by implication it should be required for leaving as well.