Putting the matter devoid of sentiment, it is entirely rational to act to maintain the prosperity of an entire nation, rather than adhering strictly to selfish factionalism. The economic engine that is Manhattan and the NYC area is part of the greatest and oldest free trade zone in human history. We all benefit from the liberty secured by this historic voluntary confederation. We are all in this together, as Americans.
Why is it, in your words 'selfish' to act in the best interests of the sovereign state of which you are a citizen?
The Republic keeps itself solvent, as framed by the original Founders' intent, by that sovereignty of those individual states, binded together by the federal government for specific and limited purposes.
If I am a citizen of Colorado and choose to live in another state because Colorado's taxes or laws appear onerous, I am free to move to New Hampshire if I choose.
Anybody has that freedom to decide what is best.
Once we become a single entity of taxes, laws, policing, judicial, etc, we have lost the Republic and our freedoms.