I'm thinking along the lines of what furio_the_nephew says. I think it all boils down to envy. He wishes his family was as loving as Bobby is with his children and his children love him just the same. He also sees Bobby as weak at times because Bobby tends to put a certain whine in his voice. Plus to boot Janice we all know is one of Tony's least favorite people in the world. But all in all I feel as if Tonys character has a certain envy towards Bobby which is why he constantly bullies him and talks down to him.

But Tony definitely gets a new found of respect for him when Bobby finally gets fed up with T's bullshit and puts the fists to him at the cabin.