From his podcast chat with Peter Attia (my emphasis):
You had to be realistic. You had to be able to look at it and say, "What could go wrong?" If I learned anything from [Jay Pritzker], I learned that if everything went too well, you could survive. The only time you couldn't survive is if it didn't go so well.
So focusing on the upside was interesting, but not productive. Focusing on the downside was what risk was all about.
And to the extent that you could quantify the downside—to the extent that you understood what the risk was you were taking—your chances of survival were much better.... I think a lot of people get in a lot of trouble because they do a transaction and they don't understand what the risk they're assuming is when they do the transaction. And what he taught me more than anyone else was [to] look at the deal and figure out: Where is the vulnerability? Where is the assumption you've made that has to be right in order for the deal to work?