What's your source for that? The Constitution is the authority which governs relations among the States, and the Constitution is silent on the matter of States' seceding.
True, but the Constitution is not silent on the fact that some powers are reserved to Congress and other powers are forbidden to the states. States cannot be admitted to the Union without the approval of Congress, cannot change their status without the approval of Congress, by implication should not be able to leave without the approval of Congress. After all, the secession of a state does not affect only that state, it can have a negative impact on the interests of the states remaining in the Union. Shouldn't they have a say in the matter?