[notahacker]

Sounds like what you really want is an introductory macroeconomics textbook that will give you an explanation of the standard models mainstream economists use.

But your example - why business cycles occur - is really more of a debate than a generally understood phenomenon. That's part of the reason why they occur. (part of the explanation is straightforward: people are less likely to borrow if they think that problems in the economy might make repayment difficult, and optimism/pessimism about the future tends to be shared by large proportions of the population. But the dynamics aren't universally agreed upon.)