The resignation is atypical, but it doesn't surprise me that he's judged himself unable to continue the task. I think a major factor is that modern health care enables us all to live well past our truly productive years, but posts like the papacy expect strength and leadership to the end. I have no way of knowing, but I wonder if he remembers the last years of JPII, when that great pontiff's Parkinson's was rending him incapacitated. Word was flying in many circles of a Vatican adrift. Perhaps B16 doesn't want to see that happen.