Of course the answer to the question 'Why do the business cycles exist? 'is not easy and it depends on the economic and political school of thought.
The classical economists (Smith, Ricardo, Mill, Marshall, Malthous, Pigou) consider that a change in the demand curve can be compensated by a corresponding change in the supply curve and reversely, and as a result the GDP deviates from its potential value which is expressed by the long-term trend. That is done through the price mechanism in the different markets (including the labour market, which classical economists found as a secondary importance phenomenon with transitive only character). As a consequence , prices consist the equalizing-corrective factor which guarantees the equality between the real and potential GDP and the avoidance of cyclical fluctuations.
The Keynesian economists believe that the business cycle is the result of the changes in the aggregate demand. When price and inflation expectations are a bit inflexible then the swifts in the aggregate demand can cause large swings in real output. Furthermore, they aspire that the price mechanism is not enough to prevent the creation of cyclical fluctuations either from the demand or the supply side. From the demand side they think that investment and GDP do not correspond sufficiently to the increase of the money supply or the interest rate. As far as the supply is concerned, it cannot corresponds properly due to the rigidity in the labour market. That means that the wages cannot decrease more than to the point where it is determined exogenous by historical, social or institutional factors.
The monetarists support that the price mechanism in the labour market is decisive for the mechanism that equalize the demand with the supply.
According to their most eminent representative, Milton Friedman, the working population has asymmetric or incomplete information regarding the conditions that prevail in the general labour markets but they have complete information only for the labour market they are working. As a result the movement from the one labour market to the other that may equalize the supply with the demand is not possible, because of the lack of information.
New classical economics are mainly associated with the work of Robert Lucas ,adopted Friedman's opinion ,regarding the lack of information in the labour market, but for this school of thought the expectations are shaped rationally , taking into consideration the current estimates for the recurrent evolution. According to Friedman's theory the expectations are shaping through the readjustment of the past errors. Provided that in Lucas theory the expectations are shaping rationally, then in the long-run the information errors approximate zero. Consequently, according to new classical economists and also Friedman the cyclical fluctuations owed to the information errors of the economic institutions regarding the 'accurate' prices that equalize the supply with the demand.(责任编辑：BUG)