Shaq has been very verbal about how he feels as though Dwight Howard doesn't live up to his full potential on the court, and he recently stated that he would pick Brook Lopez over Dwight during a round table discussion with NBA.com.  

Howard was later asked about how he feels about Shaq being so critical of his performance, and Dwight thinks Shaq shouldn't be concerned with his game, but rather sit back and just let him play.  He feels as though Shaq is a hypocrite who suffered the same kind of slander when he was playing years ago, so Dwight doesn't see why Shaq should do the same to him.

"He hated the fact when he played that the older guys were talking about him and how he played and now he's doing the exact same thing. Just let it go. There's no sense for him to be talking trash to me. He did his thing in the league. He's one of the most dominant players to ever play the game. Just sit back and relax. You did your thing. Your time is up. So, I don't really care. I don't really care. He can say whatever he wants to say."

Source: complex.com