system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The U.S. holds 18 trillion in national debt; who do we owe this money to and how does it get repaid?

I was curious due to the upcoming election and candidate talks of increasing public programs that will only put us further in debt. It seems like our priority should be to have a balanced budget and start working on bringing down the debt. I have never fully understood how our debt works. Why would individuals/countries keep allowing us to borrow money when we owe so much? How do we sell our debt? Is it through bonds from the government? If you buy a bond can you collect on it at anytime or is it usually a fixed number of years before you can cash it in?
gpt
The reason why the United States can keep issuing debt at a very low interest rate is because of the demand for US debt. It's widely perceived as one of the safest investments in the world, so much so that the finance community has historically used US short-term debt as an estimate of the "risk-free rate of return." When there is uncertainty and risk in the markets, everyone wants to hold US debt, from individual Americans, to large institutional investors, to foreign countries. The demand for US debt means that US can continue to issue new debt very cheaply, as compared to other governments or corporate entities.

Governments, like corporations, are not like people. Let me use the corporate example: why do very profitable companies still have lots of debt? It's because if the corporation can issue bonds and pay 3% interest, but generate a return of 4%, it's better that they issue debt. It doesn't work like this for individuals. And as long as they are extremely credit-worthy (which the US government is), they can issue that debt cheaply. Governments, of course, do not have a profit motive: we all know what the government spends money on: social security, Medicare, the military, etc. The government has different tools to raise revenues, including collecting the money it needs in taxes or issuing debt.

Oftentimes, you might see in your local area for a spending proposal to fail if is funded by a tax increase, but pass if it is funded by a debt issuance. This is because taxpayers would often rather their government issue debt than collect enough in taxes for the spending that they want (after all, if they didn't want the spending, they would vote it down no matter what). We want the services, and don't want to pay the taxes: ergo, our national debt. We also might want to invest in this debt ourselves, since the interest, while low, is tax-free.