(N.B This was originally a comment to a post over at Mark Shea's. I've done some minor editing.)
Patriotism has little to do with form of government and much to do with land, people, culture and tradition. But insofar as the historic culture and tradition of a people includes and is tied up in certain realities of government, these realities are owed our allegiance more so than others.
The Constitution is no more the cause of the American people than the monarchy was the cause of the French people. But the Constitution is a product of the American people just as the monarchy was the product of the French people. That's why the Vendée were better Frenchmen than any Jacobin.
I think it is more true to say that Patriotism has nothing to do with abstract principles separated from the historic and traditional life of a people. That is why I think that the preamble of the Declaration of Independence is at best a rhetorical tactic to gain support in Lockean Europe and at worst pseudo-philosophical bunk that demonstrates Jefferson's fascination with Enlightenment thought getting the better of his far nobler classical and agrarian republicanism.
The most important part of the Declaration is the section detailing the king's violation of the traditional and historical rights held by the colonists. These rights stem both from the American political tradition of deliberate consent of the governed, which had existed since the Mayflower Compact, and the traditional common law rights of Englishmen, held by the colonists as English citizens and subjects of the crown.
In my view it is erroneous to say that the United States is some grand experiment meant to be the laboratory of political philosophers. But it is just as much an error to say that the patriotism of American citizens can be completely divorced from our form of government. Both views ignore the organic tradition, as historically lived by Americans, which inform the Constitution of the United States.
P.S. One commenter says that he has the same problem celebrating Independence Day as he would have celebrating Bastille Day if he lived in France. This strikes me as an erroneous conflation of the American War for Independence and the French Revolution. Such a conflation is common, but that does not make it correct.
The French Revolution was the violent overthrow of historic and traditional French institutions in the name of abstract principles. The American War of Independence was the severing of political ties between England and the thirteen colonies due to the violation of traditional and historic rights that organically developed over time. One fought against tradition, history and organic society, the other in favor of it.