It's possible that he's really into the stories. And I think he's probably pretty excited by the new design possibilities that the Fox Engine allows in Ground Zeroes.
I just remember the Kojima that successively promised that MGS2, MGS3, and MGS4 would each be the last Metal Gear games he would work on. And I remember an interview he gave with OPM where he talked about how he saw games as a service or a tool (like a chair) more than as a piece of art, and his job was to make players happy with his games. He's always very aware of fan expectations, and you can see how he responded to the negative reaction to playing as Raiden in MGS2 by making him into a joke in 3, and then radically transforming the character in 4.
I'd like to think that he has enough power that nobody at Konami is forcing him to do anything. But since he apparently doesn't trust his staff to make a Metal Gear game without him, and since fans keep demanding the titles, he'll keep working on them.
I'm sure he's enjoying it at this point, but as long as he's working on the franchise, he can't make something outside of the military espionage genre, and I'd like to see him try something new again.