Leto tells his son about the plans to spy on his mother.
Leto will play along with Hawat's idea and act as if he openly distrusts Jessica. This way the act will seem authentic.
Leto only tells his son because he wants Paul to be able to tell his mother the truth should anything happen to him. And once you've uttered a sentiment like that, you just know something is going to happen to you.
Paul believes his father is just tired, but Leto continues, explaining to Paul why he never married his mother. Paul says he understands and affirms that Leto is a good ruler.
Leto sees one advantage to Arrakis. He says it "makes us moral and ethical" (14.36). Even if it means they must lose their self-respect to hold the planet.