Secession is connected one of the founding principles: Government by consent.
The people determine the government, not the other way around.
The colonies seceded from Great Britain.
The colonies did not secede from Great Britain. They were colonies, which meant that they were governed differently from the rest of Great Britain. All U.S. states are governed equally under the U.S. Constitution.
Everything is legal if both sides agree. Texas has always been too valuable to lose. Louisiana, not so much.
Think about it, who would bitch if New Mexico left?
The colonies rebelled from Great Britain, and fought a protracted war with Britain to establish their independence.