Why did the French help the Americans in Revolutionary war?

The French aided the Americans in the War for Independence largely out of a desire for revenge. Ever since they lost the Seven Years War and their bid for dominance in North America, the French were eager to get back at Great Britain. The American Revolution provided the perfect opportunity for this. The two imperial rivals had long been adversaries, and the French were not eager to pass up an opportunity to hurt their old foe. They understood that by losing the Thirteen Colonies, Great Britain would be greatly weakened as an empire and consequently would be less of a threat to the French in the future.
In spite of this desire to strike a blow against the British Empire, the French did not immediately throw their full support behind the Americas. Although they covertly sent some supplies and advisors to aid the Americans, the French needed to be sure that the revolutionaries stood a decent chance of securing victory on their own before committing to open hostilities. It was not until after the decisive American victory at the Battle of Saratoga in the autumn of 1777 that the French felt confident enough to openly support the rebellion.
https://history.state.gov/milestones/1776-1783/french-alliance