Here is Blair's answer to that million dollar question - for the thrill of an "explosion of irresponsibility".
"There is the moment of encounter, so exciting, so naughty, so lacking in self-control.
"Suddenly you are transported out of your world of intrigue and issues and endless machinations and the serious piled on the serious, and just put on a remote desert island of pleasure, out of it all, released, carefree.
You become a different person, if only for an instant, until returned back to reality," he writes in his best-selling memoir 'The Journey'.
"It's a strange thing, politics and sex. People have often said to me that power is a kind of aphrodisiac, and so women politics still being male-dominated would come on to politicians in a way they would never dream of with anyone else," Blair says.
"I totally understood the desire to escape. And it's nothing really to do with how happy or otherwise your marriage is. It's an explosion of irresponsibility in an otherwise responsible life," he writes seeking to justify the behaviour of his former deputy Prime Minister John Prescott, who had an affair with Tracey Temple, his diary secretary.