It's like Mean GirlsJeepers, the guy just wants to make some movies, have lots of babies and make screaming monkey noises with Angie when the kids aren't around. But eternally disgruntled Sean Penn will not allow that to happen.
Sean and Brad are working together on Tree Of Life, a new adventure film. Brad is taking over Heath Ledger's role, and perhaps that has something to do with it - Sean resents Brad for replacing his fallen friend. More than likely, though, it's that he felt compelled to reconcile with Robin or risk losing half of everything he owns. The couple returned to their status quo, and Sean was left wanting to kick the dog. Unfortunately, no dogs were available, so he's taking it out on Brad.
Sources say their differences are purely professional - simply a difference in working styles - but others point to the close relationship Sean and Robin have with a certain manly former sitcom star. Jennifer's big masculine hands are stirring a lot of pots, and it doesn't look like she or her friends are in any hurry to give up the Brad-bashing any time soon - especially with another Jolie-Pitt baby about to arrive.