The value of anything depends on demand; the demand for USD comes in part from the need to pay taxes to a government that will only accept USD (and more general

The value of anything depends on demand; the demand for USD comes in part from the need to pay taxes to a government that will only accept USD (and more generally, from the fact that in the United States, the law only recognizes USD as money -- so things like debts, torts, etc. wind up being measured in USD, so anyone who has a legal obligation to make such payments will add to the demand for USD).