Back in the days when I was selling cars, we would do a walk around of the customer's trade-in, and pointed out each defect, even small dings. The more we criticized it, the more the customer was willing to pay for the new car. Sure, it's uncomfortable, and it makes the customer feel bad, but it helps get a better deal.
It's like Trump is pointing out to Mexico how weak of a position they hold. Just be glad he's on our side! The last guy seemed to give everything away!