Political revolutions can be started for a variety of reasons, but perhaps the greatest throughout history as been the desire to overthrow or greatly alter the government in charge of a state or country.
One very important factor that has been observed in history many times is that people are dissatisfied with the current political climate and the situation. Often a dissatisfaction with the political climate/situation is caused by bad or worsened conditions in which people themselves live in. For example, prices of food drastically rising because the country is being run in a bad manner.