This will largely restate what others have said, but here’s my 25%.
To me, there are two primary justifications for a progressive tax scheme. The first, and more compelling to me, is argument from a position of morality or ethics.
Since I’m typically pitching arguments on this topic to conservatives, I pretty much skip right over issues of morality and ethics.
The second is from a pragmatic standpoint. Even if you believe that a rich person “made” their own money*, there is a system in place that allows them to be rich. They don’t have to spend much money defending themselves from marauding hordes. There is a system of laws and justice in place to help protect them. There is a financial system in place to help them continue to generate wealth from their current wealth. It would seem to be a pretty good position to be in, so the rich person seems undeniably to have a vested interest in making sure the system stays in place.
Now, if that system can be maintained by everyone paying whatever the poorest person can similarly pay, super! You’re in a remarkable system that provides a great deal of benefit from the most meager of means.
Since this doesn’t occur in reality, it means that a progressive tax system is more pragmatic. It’s the more likely to allow you to have and enjoy wealth and to maintain the system as you prefer it. To me, it’s a bit like a baseball park in which there is a single ticket price of $10 for the bleacher seats all the way to the luxury suites. Sure, you could set things up like that, but if you want to be the New York Yankees and not the Pittsburgh Pirates, it ain’t gonna happen.
*As to the “making” of money, I also argue that those who have become wealthy in America have done so by taking advantage of the system in place. In addition to the features that allow one to remain wealthy, the system offers the benefits of transportation and communication infrastructure, an educated populace, public health protections, and so on and so forth. Not only does the entrepreneur directly benefit from these things, but they also provide him or her with a good pool to draw employees from, and critically, a market of consumers that allows him or her to benefit in order to make that money.
I don’t begrudge anyone the opportunity to benefit in such a way, but it certainly pisses me off when people turn around and pretend that those benefits were solely the result of the efforts of the individual. If that person didn’t educate everyone, build the roads, provide the police services, dig the sewers, electrify the households, write and enforce the laws, and again so on and so forth, they did not do it all independently of the system they ought to be obliged to continue to support.