It almost feels too soon to start the post-mortem on John Key's Prime Ministership, which will come to an end in one week.

The question everyone is asking right now is why?

He is still immensely popular after eight years.

He stood a good chance of winning a fourth term next year.

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His Government continues to have energy despite being in its eighth year.

Why would the person with the most glittering political career in modern times suddenly declare he is not a career politician and resign to try something else.

"I gave it all I had; I left nothing in the tank."

For a decision that he says was made in September, Key delivered his carefully written bombshell with trembling emotion.

He relaxed more when facing questions from a shell-shocked Press Gallery in the Beehive Theatrette.

But the emotion returned when talking about his family, his wife, Bronagh, and Steffie and Max, the kids who have grew up with the country watching.

By the end of the 40-minute press conference, other possible theories abounded on social media such as health issues, and Bronagh demanding it.

They will abound because what Key has done defies political gravity.

But Key insists that the time is right, that he could not have completed a full fourth term, and he could not have misled the public the public by pretending otherwise.

It is true that that is not the way he operates, at least not the way he operates comfortably.

In the absence of evidence to the contrary, Key can be taken at his word.

He has almost always been a winner in life.

He wants to go out as winner on his terms.

Ever the optimist, Key gave a rendition of someone who genuinely believes that National can do as well without him as they can with him.

It is doubtful that Key's caucus shares the quite the same optimism.

The quietly thrilled atmosphere of the Opposition wing does not share his optimism.