Why did the French join the American revolution ?

Because France was Allies with America.

Explanation:

At the start of the war, France gave supplies (like gunpowder, clothing shoes etc) to the Continental Army. In 1778, France officially became allies (friends) with the US through the Treaty of Alliance. This made France become involved in the war.

i belive the continental association was created to boycott all contact with british goods.

i also belive it was in december 1, 1774