There is this thing called a zero lower bound. Then there is this thing called liquidity preference, which is basically the minimum fee someone accepts to part with liquidity.
Add those two together and you will get a minimum positive return constraint on capital.
Capital can only pay a return if it is fully utilized. This means there needs to be enough demand to saturate the machine.
You can never have any form of excess capacity, because excess physical capital does not generate any returns. It actually loses you money, which is incompatible with the positive return constraint.
Since the expected return is positive, the lender has to pay back more than he borrowed, meaning that he has to have more revenue and more revenue is earned by selling more products.
Now, this alone doesn't explain the reason why you have to keep going though.
The question is, when someone demands positive returns and those returns are artificially anchored against zero thanks to the ZLB, why is this bad.
Well, think about what happens if nobody borrows the money. That means the money is still waiting for a borrower and it is not turning into demand. Now there is money missing in the economy. The rest of the money has to circulate faster to compensate the loss of money but it doesn't, because everyone is thinking the same thing.
The problem here isn't on the business side. The problem is that someone decided to use an unsigned integer for something that can be negative.
You need money for daily transactions. You don't care about the nominal value of the money as a "store of value", you care about it as a way to turn your illiquid work output into a liquid voucher you can redeem at any store.
Since modern society depends on money, having the money system degrade is similar to an internet or electricity outage.
So what happens instead is that the government takes on more debt because that is the logic of a zero lower bound.
It also forces the central banks to do QE, because there is nothing you can do at zero.
In both cases, the system is being flooded with the same defective money. Someone will hold onto it, due to their liquidity preference and the lack of negative interest rates.