Marginal utility of consumption and interest rates
The incentive fee referred to here is the interest rate.
The wealthier you are, the less you value marginal consumption which means the utility from extra consumption will be lower in the future (if there is economic growth). This implies consumers would rather consume today rather than in the future (when they will be richer due to economic growth). To incentivize them not to consumer too much today, we need a higher interest rate so they save today and instead consumer in the future.