When interest rates are higher than GDP growth, most people won't be able to pay their debts? How can people pay high interests from economic activity, if the economy just doesn't grow that much? This makes a casino-economy where most people come to "play" (by getting debt), yet only a few will profit in the long run. Most will lose, ending up with less than they started with?
Go to your local public university, get the syllabus for macroeconomics and statistics and buy the required text's and learn this shit.
If you fully understand the terms it's intuitive. If a nation/person borrows at a greater rate than their income/wealth increases it's not sustainable.