Full Idea
The tragedy is when two 'nice' players defect because they misread the other's intentions. The puzzle of the prisoner's dilemma is how such good intentions pave the road to hell.
Gist of Idea
The tragedy in prisoner's dilemma is when two 'nice' players misread each other
Source
William Poundstone (Prisoner's Dilemma [1992], 11 'Howard's')
Book Reference
Poundstone,William: 'Prisoner's Dilemma' [OUP 1992], p.226
A Reaction
I really wish these simple ideas were better known. They more or less encapsulate the tragedy of the human race, with its inability to prioritise the common good.