There are many people who support progressive tax, since they feel it is only fair that those who make more should contribute more to the government. (wiseGEEK)

The modern theory of optimal income tax progressivity begins with the utilitarian principle, but views the issue as a trade-off between the social benefits of a more equal distribution of after-tax income and the economic damage imposed by highly progressive taxes. (econlib.org)

The progressive tax system, and the nation's fiscal system more broadly, have historically played an important role in expanding opportunities for all Americans while reducing inequality. (Brookings)