Checks and balances is the answer.
When interest rates on borrowed money are lower, it becomes cheaper for individuals to borrow money, as they must pay less additional money as interest. Thus, they tend to borrow more money and use it to purchase more things. The opposite occurs when interest rates increase.
When interest rates on invested money are lower, people make less return off of their investments, so they tend to invest less. Again, the opposite occurs when interest rates increase.
