I'm not sure I agree (or at least, not completely). Self-determination is sovereign, but there is also an issue of size. Say, for example, Louisiana secedes from the US: what would happen if New Orleans wants to secede from the newly independent Louisiana (as an exclave of the US or its own city-state)? Should it be able to? The Greater New Orleans Area has more than 1 million people, about 1/4 of all inhabitants of Louisiana, so hardly a small community in comparison. I dislike making slippery-slope arguments, but this is something you have to consider.
You get stories like this down and again. Staten Island has threatened to secede from NYC, and the town of Killington, VT threatening to join New Hampshire despite being 40 miles away and landlocked by Vermont towns. It's largely posturing on behalf of someone who wants to make a bold statement and get re-elected.