This happens because lenders determine how much they can lend on the basis of the borrowers’ 1) projected income/cash flows to service the debt, 2) net worth/collateral (which rises as asset prices rise), and 3) their own capacities to lend. All of these rise together. Though this set of conditions is not sustainable because the debt growth rates are increasing faster than the incomes that will be required to service them, borrowers feel rich, so they spend more than they earn and buy assets at high prices with leverage.
