Because that change in behavior is what lets markets work. With an inflationary currency, people want to create more wealth which is a net benefit to a society. With a deflationary currency, investing in the real world loses money, which disincentivizes projects that make the world better.
The problem with these purely verbal arguments is that they can be made to sound convincing either way.
E.g. Inflation is good because it encourages people to invest in things, creating jobs. However, inflation is bad because it encourages people to take unnecessary financial risks (like borrowing or lending money) when they could instead save it for a rainy day.
Saving the money for a rainy day is what strangles the economy. The economy's health is, to an approximation, the rate at which people spend money.
Slow the rate at which money flows through the economy, and bam, welcome to a recession/depression.
It's not a question of morality, as much as it's a question of mathematics. You may consider saving to be virtuous, and debt to be sinful, and sure, fine, that is your value system, but the only reason any of us have work is because we all spend, as opposed to save.
You can envision a society where people mostly work for themselves, where this wouldn't be an issue. Unfortunately, modern society relies on specialization, and I can't make most of the things I need by myself.
