It's bad if you owe money. And many people do. They don't want to owe more value because the currency becomes more valuable. (They would owe the same number of dollars or bitcoin or whatever, but they'd owe more value, because each dollar or bitcoin would be more valuable.)

And then comes the second-order effects. Each debtor owing more value means more of them can't pay, so more default. This leads to things like houses reverting to the lenders, who then sell them for what they can get. This reduces the price of those assets, which causes additional deflation.

When this cycle really gets going, it crushes lots of people - even responsible people, whose only mistake was getting a mortgage when times were good, and maybe a car loan and a bit on their credit cards.