When a depression starts, the society it effect usually has been on capitalism for a couple of generations. Previous methods of production die off with the older generation. The new methods are failing because of the collapse of the market and people cannot produce a livelihood. Since businesses cant afford to employ workers, large amounts of layoffs occur. This starts a downward spiral, as people loss their sources of income.
People are unable to be good consumers, they have no money, thus more layoffs occur.
Capitalist believe that in order to succeed in capitalism, the market must expand. But this expansion cannot last forever. Just like a spinning wheel, friction will enviably make it stop. The fact that people become wealthy off capitalism its what eventually crashes the market. Wealthy individuals are less dependent on the system, and thus less inclined to work.