As The Economist explains:

The financial system--What went wrong:

The American financial-services industry's share of total corporate profits rose from 10% in the early 1980s to 40% at its peak last year. Its share of stock market value grew from 6% to 19%. These proportions look all the more striking—even unsustainable—when you note that financial services account for only 15% of corporate America's gross value added and a mere 5% of private-sector jobs.