For the first hundred years of American history, there was one cause of public debt: war. The United States of America started in debt, and it paid its bills. There wasn’t another spike in debt to Gross Domestic Product ratio until the American Civil War.
At this point, you might ask what is the debt to GDP ratio? It is the amount of our public debt compared to the size of our economy. That ratio is the number that economists care about.
It is not hard to understand. Your bank will lend you more money if they know you have a high income and high long-term earning potential. The bank will lend you less money at a higher interest rate if they know you have a low-income and low long-term potential.