For some people, it's in their nature to never be pleased with what they have and thus, they always aim for more and want more. It's in their nature and it is my opinion that Mr. Murdoch will never be satisfied with what he has. It is in his nature and as we all know, nature is unchangeable.
There are some people in this world who are never satisfied no matter how much they have. I would argue that it is part of the human condition. To always be striving and working for more. I think that some people have a stronger desire than others and that is what we see here.
I believe he is bulking up due to the rejection to his bid on time Warner. He already has 20th Century Fox and just wants more. He will keep going till he gets Time Warner. When you are working in Television industry and everyone is fighting each other, the more you have the better chance of success. So I believe that he will stop when he gets Time Warner.
Despite all the long years of controversy surrounding Rupert Murdoch, powerful men tend to want only one thing, which happens to be more power. Mr. Murdoch seems like the wants to control all aspects of media to push his own agenda and get people to think like him. That's power.