What were the political and social causes of the French Revolution?

The central motivation for the French Revolution was inequality, in a few different senses.

There was political inequality, as the monarchy granted extreme power to an individual family while denying political representation for the vast majority of the population.

There was economic inequality, as the distribution of wealth had become so extreme that a handful of people were enormously wealthy while the majority were in poverty.

There was social inequality, as the social classes (royalty, nobility, peasantry, merchants) were rigidly defined and often in conflict with one another.

During the same period, a rising educated middle class was forming, and they became involved in learning and spreading ideas of liberty and equality from what we now call the Enlightenment. They were growing increasingly intolerant of these long-standard injustices, and increasingly confident that they had the power to rectify them. (This is a general pattern, actually; revolutions happen not when oppression is at its worst, but when it just barely starts to get better. When oppression is at its worst, the inequality seems inevitable and revolution seems impossible. But when people see that things could have been better all along, they often rise up to make it so.)

But these inequalities might have found some resolution without a violent revolution were it not for two major triggering events.

The first was an economic crisis, triggered by irresponsible budgeting by the monarchy as well as poor crop yields that year. There was widespread unemployment and food shortages.

The second was the American Revolution, which actually had two major effects. The first was that it was very expensive---a lot of the irresponsible government spending by the monarchy was actually involved in supporting the war in the United States. The second was that it was successful---the US successfully broke away from the monarchy in Britain and began to set up a new democratic government. So, the French revolutionaries thought, "Why can't we?"