Why is this the case about the worth of a dollar?
A dollar you own today has zero risk of not being delivered tomorrow.
You can invest that dollar one day sooner.
If there’s a general expectation of inflation, it exchanges for more today than tomorrow.
Think of a dollar today as one commodity and a dollar tomorrow as a separate and distinct one. There is a difference in our marginal preferences and that is where the interest rate comes from.
Just as both of us can be made better off trading blueberries and strawberries without actually creating anything new, the difference in our inter temporal marginal preference will reward the lender with a return even in the absence of risk.