Charlie Gibson Retirement Reason?

Just over three years after taking over at World News Tonight, Charles Gibson has decided to retire from full-time employment. What is the reason of his retirement ?
The official announcement came today after several weeks of discussions with ABC News President David Westin.
But I didn't hear a reason. Is Charlie Gibson retiring for good?

He didn't give a reason for his retirement.
Charlie Gibson's closing note from the end of tonight’s broadcast is below:

"As has been mentioned a time or six over the past week -- this is my last broadcast in this chair. And it's hard to walk away from what I honestly think is the best job in the world -- but my parents taught me you should understay -- not overstay your welcome."

I also want to know the reason. Charlie Gibson has a disease, maybe it has something to do with that.
I will miss Charlie Gibson.