Balaamsass2: He always lived below his means
This is, IMO, one of the underrated keys to happiness. It means that you're not just debt free (with the possible exception of a mortgage), but that you're not always dancing on the edge of a knife with regards to monthly expenses. And that you're probably in good shape to deal with the unexpected expenses that crop up from time to time.
I've often told my friend's kids about the importance of living below your means and saving money. The extra peace of mind you get, and the lack of accumulated stress and anxiety, are worth more than all of the fancy stuff you could've bought (and gotten into debt for).