What was the main reason for …

Despotic rule of Louis XVI: He became the ruler of France in 1774. He had drained the financial resources of France in wars. For the cost of his regular extravagant expenses, he increased taxes which were paid by the third estate.
Division of French society: The French society was divided into three estates; first, second and third estates, respectively. Among them, First and Second Estates had 10 per cent of the total population and were the clergy and nobles. They enjoyed all privileges. Rest of the 90 per cent was of the third estate, which paid all kinds of direct or indirect taxes. This kind of discrimination led to the revolution by the third estate.
Rising prices: The population of France had increased. This resulted into more demand of food grains. So the price of bread rose rapidly, which the poor could not afford to buy. So the gap between the rich and poor widened.
Inspiration of the philosophers: Philosophers like John Locke, Jean Jacques Rousseau and Montesquieu propagated the ideas of having a society where people would enjoy freedom, equal laws and equal opportunities. Their ideas inspired the people of France to realise their dreams.
Role of middle class: The 18th century witnessed the emergence of social groups termed as middle class, who earned their wealth through an expanding trade of the manufactured goods, being exported. There were other professionals also like lawyers, administrative officials who were all educated. They believed that no group in society should be privileged by birth.

Following are the social, economic, political and intellectual causes of the French Revolution:

Social -  In France, the clergy and the nobility formed the first two Estates and were the most privileged classes in the French society. They were exempt from payment of taxes to the State. On the other hand, the Third Estate that consisted of peasants and workers burdened with excessive taxes with no political and social rights. As a result, they were full of discontent.

Economic - As a result of a numerous wars waged by Louis XVI the State coffers were empty. The situation was made even more complex by France’s involvement in the American War of Independence and the faulty system of taxation.

Political - The Bourbon king of France, Louis XVI was an extremely autocratic and weak willed king who led a life of obscene luxury. This led to a lot of disenchantment among the masses who then were leading life of extreme poverty and widespread hunger.

Intellectual - The 18^th century was marked by a conscious refusal by French thinkers of the ‘Divine Rights Theory’. Philosophers like Rousseau, rejected the paradigm of absolute monarchy and promulgated the doctrine of equality of man and sovereignty of people.