While the French were instrumental in assisting the American Revolution, it was an exercise in Realpolitik for France, since the revolutionary war tied up a lot of British ships and troops which might otherwise have been used against France. Since France has also been soundly drubbed in the Seven Year's War, revenge is also a strong motive.
Not long after "These United States" gained Independence, America was involved in a protracted "cold war" with France, and in the 1860s, France openly supported the Confederacy.
The French, like everyone else, are driven by their perceived self interest. Unfortunately for France, they seem to have much bigger "self interests" then their economy and population is able to support, still nursing dreams of their lost Colonial Empire and the glories of Napoleon.