The US Constitution is silent on the issue of secession. Live it, learn it, love it.
It is not silent on the broader issue of controvery between the states, or between the states and the federal government.
Such controveries are to be resolved by law, per Article III, with the supreme court as original jurisdiction. Because of that requirement, secession at pleasure is unconstitutional.
The Declaration of Independence is not. It is quite clear about it.