So, in general "incomplete contracting" is one of the biggest sources of tension and conflict in the world. What this means is that a lot of negative situations arise because we need to trust other people, but we can't trust them.
Suppose you're pointing a gun at someone and they're pointing a gun at you. Neither of you wants to die or kill. So you ask them to lower their gun and you'll lower yours. But why would they lower their gun, because if they do, they'll shoot you. The mechanism you need to solve this is a way to force you both to stick to the deal. That's a contract.
Treaties are contracts. Ideally every country would set policy that benefits them maximally and doesn't care about anyone else. But then no one would ever agree to do anything. So treaties are about give-and-take. And the reason you can believe that treaties will work is that, presumably, a) politicians face a major domestic cost for breaking treaties; b) third-party international monitors like the WTO can adjudicate disputes.
Okay, suppose NAFTA is bad, and you want to withdraw. The problem is now why would any country, ever, trust you on a treaty again? Knowing that you'll just break it when you feel like it's not going your way. The treaty is no longer a contract. And we're back to people pointing guns.
The problem with "the Art of the Deal" is that it's just "the Secret". "If you want something, you'll get it". Yeah, except that's not how being an adult works.