If I have 10 pounds of blueberries and you have 10 pounds of strawberries there will be a natural price for one in terms of the other based on our different marginal preferences for the two commodities.

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.