It's true because it's all relative. Like Einstein's theory of relativity, which states that your location and place in space/time is relative due to constant movemeant, your money buys you different things at different prices in different areas. Your money is constantly relative. Today you live like a god in Thailand tomorrow like a pauper in NYC. This not only includes space, but also time. How so?
Well would you rather be middle class today or John D Rockefeller 100 years ago? I'd take middle class today. Better homes, electricity, indoor plumbing, better health care, the list goes on and on. So if for some reason you don't feel rich. Think, "Relative to what?".
Americans over the past 24 months have almost double or increased their wealth/income by 50%. Doesn't seem like it? Go to another country and see for yourself. No greater example of this is probably Russia. You can now go there and buy items for 25 cents which would have used to have cost you a dollar. Relative to the Russians you're at least 4 times richer.