It was evident in his biography, John Key: Portrait of a Prime Minister, that his threshold for tolerating failure is low.
In 2012, after a difficult but not disastrous year, he talked to wife Bronagh about whether he was still committed to remaining in the job. And she was stronger than him about staying on and not be seen to be "running away", as he put it.
He has said he will stand again in 2017 because that's what leaders have to say until they change their minds.
But nobody would be shocked if Key changed his mind if his popularity waned, given that his popularity sustains his political drive.
If it happened, it would not happen soon because he would want to recover his respect rather than slink away.
There is no suggestion of a leadership challenge. But Key is losing his Midas touch: he declared that Winston Peters had "zero" chance of taking the Northland seat just weeks before he took it; he is leading the charge for a change in flag which is increasingly running against the current, according to the latest Herald-DigiPoll survey. And the ponytail incidents have relegated him to the ranks of the emperor with no clothes in a caucus which hero-worshipped him.
These are all results of personal failings, vanities and arrogance.
Cabinet ministers know that any one of them would have almost certainly been sacked by now if complaints about ponytail pulling had emerged about them.
Last term, Judith Collins and Steven Joyce were regarded as the great leadership rivals.
As a genial and competent super-minister, Joyce was seen as the leader-in-waiting in the event of Key deciding to have a managed handover during a term in Government.
As a bit of a brawler, Collins was the strongest contender for the leadership if the party had found itself in Opposition.
She made poor judgments that eroded her public popularity and last year resigned as a minister after emails emerged suggesting she had had a role in undermining the former head of the Serious Fraud Office.
She has been lying low, popping up to blow the whistle on water reforms the Government is discussing with iwi leaders, and making new friends with backbenchers.
There is no doubting her political skills, as was evident last week in the heat of the ponytail saga.
In an interview on the Paul Henry show she was able to clearly say that what the Prime Minister did was wrong, while giving no sense of disloyalty to him.
Most of the Cabinet lack the skills or the guts to navigate such a perilous subject.
Self-doubt has never been one of Collins' weaknesses and she has always been one of the relatively few in the party who cares about gender issues.
Former High Court judge Lester Chisholm, who is no pushover, found there was nothing to suggest Collins had acted improperly regarding the Serious Fraud Office claims.
When Collins was cleared, Key said there was no vacancy in the Cabinet, but indicated she would be back when there was one.
In the meantime, there has been no sense of injustice that a wronged woman has not yet been reinstated to the Cabinet because, while she resigned for something she didn't do, she didn't resign for something she probably should have - her dealings with Oravida.
But there will be a vacancy soon. When Trade Negotiations Minister Tim Groser goes to Washington later this year to replace Mike Moore as ambassador, Todd McClay will step into Groser's trade shoes.
That will leave Inland Revenue vacant, a perfect portfolio in which to ease Collins back into the Cabinet - not a portfolio that requires a strong public profile, but important enough to need a firm and competent hand in reforms ahead.
She could again be a contender for the leadership were the party to be forced into Opposition in 2017, but the competition would be stiffer.
Paula Bennett has been groomed by Key, Bill English and Joyce for greater responsibility and would be a leadership contender, as would Simon Bridges.
Amy Adams and Jonathan Coleman could be anyone's deputy.
Chris Bishop and Todd Muller are the most impressive of the current intake and by 2020 could have worked their way into contention for at least senior roles, if not deputy.
It is difficult to see English, the deputy leader, or Joyce getting any satisfaction from leadership in Opposition.
Their only path to leadership would be if Key decided to step down during this term.
And while Joyce once had the halo of being the chosen one, the balance of power has changed.
If English wanted the job, he would almost certainly get it.
It is a long way from his disastrous experience as leader shortly before and after the 2002 election.
He has managed the economy almost back to surplus and effected a profound change in the way that governments - this one and future ones - will look at public services.
He has made fewer mistakes than Joyce, earned more respect and cultivated more supporters in the party.
Bennett would be his deputy and Joyce would be his Finance Minister.
Were that combination to lead National into the next election and fail, then Bennett, Bridges and Collins would slug it out for the leadership.
The DigiPoll survey this week showed no significant difference between the half of the polling before and after revelations that the Prime Minister was a serial ponytail puller.
It may remain the case, but the damage of such issues can increase rather than diminish the longer they are drawn out.
The full impact of David Cunliffe's apology, when Labour Party leader, for being a man did not occur in the few days after he said it but in the weeks and months after it.
John Key will be back in the country this weekend for the first time since the revelations and in the House next week for the first time.
It is by no means an issue that is done and dusted.
