The Mr And Mrs Smith co-stars embarked on a romance earlier this year (05) and their relationship is so serious, Pitt has filed to legally adopt Jolie's two children.
But Stern believes Jolie will soon get bored of the actor, who recently divorced former FRIENDS star Jennifer Aniston.
The radio personality says of the couple, "They belong together. They're the two best-looking people on the planet, aside from me and (my girlfriend) BETH. They should be mating.
"(But) she probably wants to get rid of this guy. I bet he's a bore after you sleep with him a couple of times."