Money represents human productivity, but the interest on money represents risk. This means that the lender collects interest because that represents the risk that they assume in departing from their money. Meanwhile productivity fluctuates naturally and can be affected by a many external forces.
The problem is that risk can never be negative, therefore interest rates can never be negative – that is called “breaking he buck”. Risk is a measure of volatility, or, “deviations from what is considered normal”. While there is certainly good deviations and bad deviations, there can never be a “negative” deviation from normal – it is a mathematical impossibility, a glitch.
The result is that productivity must always be driven up and up and up – sometimes in unnatural ways, such as forcing consumption. Constant production is unacceptable – it must always increase. Vacations, free time, family time, and leisure are not acceptable. What if we had a currency that could accommodate a negative interest rate?