I was debt-free once. I had paid off my student loan and had no consumer debt. I thought I would immediately start spending my suddenly-increased disposable income but I didn’t. Instead I found that I was very cautious with my money until I made the choice to go back into debt via a mortgage.
It turns out that I was like most debt-free people. It takes a lot of work to become debt-free and those lessons tend to stick with people.