The kicker is the first sentence. Today's workers pay today's retirees. The payments are designed to ensure a consistent, inflation adjusted standard of living for retirees. If the ratio of retirees to workers stayed the same, the payments would. But when there fewer retirees per worker, workers pay less, and when there are more, they pay more. It's the system's fatal flaw.
The problem now is that baby boomers paid much lower Social Security taxes than us, but they still need to receive the same standard of living that we will. You can see the historical rates here:
http://www.taxpolicycenter.org/taxfacts/Content/PDF/ssrate_h...
They paid a lower % with a much lower cap, even adjusted for inflation. They did so because the retiree to worker ratio was lower. If they paid the same percentage and cap that we do now, Social Security would be sitting on enough money to cover the shortfall in the future.
In 20 years, that ratio will be much higher than today, meaning the workers of 2018 would have to pay far higher taxes still for the system to fund itself. This would, of course, be unbearable (it's already close) so the government will have to seek other solutions, such as borrowing money or printing.