After eight years even the best leaders start to fray at the edges. Bad habits get worse, ambitious underlings get impatient, or luck runs out.
Not so Angela Merkel. Opera lover, physicist, political tactician par excellence, baker of cakes, stateswoman: the "Grey Mouse" has metamorphosed into "Mutti" (Mummy), the most powerful woman in the world.
She has just emerged from an election campaign that appeared about as taxing as an afternoon nap. Her impassive, guarded demeanour is unruffled, her popularity unprecedented, and Germany has never been happier to settle into another four years in Mutti's maternal embrace.
She is the longest-serving of Europe's leaders. Merkel's contemporaries from eight years ago have all fallen: to crises or fatigue or the natural political cycle. The last to go was the prime minister of Luxembourg, who resigned in July after a spy scandal.
Wherever she goes, Merkel is the centre of attention, the hub of influence.
"In a room with the other European leaders the biggest jostling for attention is always around Merkel," says a high-ranking Berlin political insider. "Whether it is an industry function, or an official event, there is an almost unseemly surge as people try to get her attention, to 'enter her presence'."
It wasn't always so; Merkel used to be mocked for her plainness, but her hearth and home air is now seen as maternal rather than unsophisticated.
In his authorised biography, Stefan Kornelius paints Merkel as "a political loner", an outsider who came to power through her abilities rather than ambition. Born in 1954, the daughter of a Protestant pastor and a teacher, she grew up in the charming little town of Templin, in East Germany. "No shadow darkened my childhood," she once said.
At school she excelled at Russian and mathematics. She would rush out of the house in the morning and visit museums. A school friend remembered her to the BBC as "very quiet, a rather plain and mousy girl, but very nice and friendly and extraordinarily intelligent". Her maths teacher recalled a student who never gave up on a problem.
Merkel's political awakening was the rise of the Berlin Wall in 1961. Her mother cried all day. Under socialist dictatorship she learnt how to play her cards close to her chest. "I wouldn't lead my life in opposition to the system because I was scared of the damage that would do to me," she said in 2005. Her parents encouraged her to join the communist Free German Youth. She excelled at university, wrote a PhD in quantum chemistry and was the only woman at East Germany's Academy of Sciences. She was living in Berlin when the Wall came down but spent the evening, as usual, at the sauna, then went for a beer with some friends.
But behind closed doors she had been developing her political beliefs with fellow academics. She joined the Democratic Awakening party, and her organisational talents put her on a dizzying climb to influence. Before long she was an unlikely deputy spokesman for East Germany's government. The party leader remembers her in a baggy skirt, Jesus sandals and a cropped haircut, "a typical GDR [German Democratic Republic] scientist".
When Germany reunified she joined the Christian Democratic Union and was picked by Helmut Kohl for his cabinet. He called her his "little girl" and promoted her well. But after the party lost government she turned on him in an audacious act of patricide, and found herself the party leader, battling Gerhard Schroeder.
Pre-Merkel, German politics was Teutonic: big, strong men with broad personalities. But Merkel refused to play their game.
"At the beginning men tried to humiliate her, they were very authoritarian towards her," German cabinet minister Ursula von der Leyen told the BBC. "She let them have their way, she was very soft, answering in a low voice. Men couldn't cope with it at all because that was not typical behaviour. She is not pompous, she is very quiet. I think that's what people like about her."
A perfect example was the televised debate in which Schroeder tried to intimidate Merkel into a junior position in her first post-election coalition. She let him bluster and brag, played out the rope, and with a slight smile let him hang himself with his own machismo. Weeks later she was Chancellor.
Merkel is married to a scientist, her second husband, and carefully guards her personal life. But details have gradually come out. She is said to be terrified of dogs and horses. She loves Wagnerian opera, supposedly does her own grocery shopping, and carries a battered old Nokia phone from which she compulsively texts her inner circle.
Stony-faced in public, averse to excess of emotion, she is said to have a dry, wry sense of humour, and a talent for mimicry.
The Berlin insider has nothing but praise for her public persona.
"She demonstrates a mastery of policy, of the detail and the tactics," the insider says.
"She's is extraordinarily able to absorb a brief. She never loses control, she always knows what is coming next - an amazingly nimble and agile thinker."
She thinks about things, to put it mildly. Even her characteristic hand gesture, the "Merkel-Raute", a diamond-shaped resting of the hands over the stomach was not an accident. "There was always the question what to do with your arms," she once said, so she talked to a counsellor and chose the Raute because "it contains a certain symmetry".
Merkel is often compared with Margaret Thatcher - especially now she is set to surpass her as Europe's longest-serving female leader. On the surface there is much to compare: two scientists who became conservative leaders.
But the difference is stark. Thatcher is famous for saying, "The Old Testament prophets did not say, 'Brothers, I want a consensus'. They said,'This is my faith. This is what I passionately believe. If you believe it too, then come with me'."
Merkel would never think, let alone say, such a thing. Consensus is her superpower.
"She never polarises, she always makes sure that people save their face," von der Leyen told the BBC. "She knows you always see people twice during your life, therefore she really tries to keep people in a consensus. This is sometimes slow."
Merkel addresses this slowness in a TV interview, revealing that when she is faced with a tough decision she tries to deliberate all the options - "running through scenarios and not simply theoretical experiments in my head, but I also try to live with that decision for a while."
But this approach has left her open to accusations that she stands for nothing - that her lack of ideology leaves her paralysed when there is no obvious right or wrong answer. "She is power incarnate," one critic told the BBC - but conserving that power was more important than her decisions.
In 2011, Franziska Augstein, writing in the London Review of Books, dubbed Merkel "consistently inconsistent". There was the Libyan intervention - despite her support for the US and NATO, Germany abstained at the UN.
There was her U-turn on nuclear power after the Fukushima disaster. When running for chancellor in 2005 she opposed Schroeder's plan to abandon nuclear energy. After Fukushima she spent a day absorbing information on nuclear safety, then made the bombshell announcement that Germany would abandon nuclear power generation.
Merkel was widely criticised for dithering in the face of the eurozone crisis. Some interpreted it as a personal crisis: her Lutheran disdain for those who let themselves get mired in debt contrasted with her core belief inherited from Kohl, that a unified Germany requires a unified Europe.
Others argued that it had to be done slowly, deliberately, in order to bring the German public with her.
Markus Feldenkirchen, political commentator at weekly news magazine Der Spiegel, does not see Merkel's slow approach as a virtue, rather as missed opportunities.
"She can talk for an hour without saying what's going to happen in the next four years. She doesn't say, 'I have a plan', and she's not fighting for it. She is more happy if there is an opportunity to do nothing. This is not healthy for Europe."
But Merkel is not a complete chameleon. She was shaped by the Cold War, and her fundamental belief is in personal freedom.
In her first public speech as Chancellor, she explained what moves her: "For me, when the Wall came down it gave me enormous opportunities. I want to repay my country and give everybody a fresh start."
She is, indisputably, completely committed to Europe - and its currency. "If the euro fails then Europe fails," she has said.
No one succeeds in politics without making enemies. A hostile biography painted her as Die Patin, a conniving mafia-like godmother.
Another, The First Life of Angela M, claimed that in East Germany she had been much more active and senior in promoting communist ideology than the official version of events would have it.
In austerity-hit Europe she is maligned as a jackbooted German. Devil horns and Hitler moustaches are drawn onto her face and carried in anti-austerity protests.
Though she is wildly popular now, only two years ago her turbulent, divided coalition was dubbed one of the worst in Germany's history.
Four years is an eternity in politics. And Merkel faces serious, immediate threats to her power. Next year there will be a European election which is likely to bring more eurosceptics into Brussels, white-anting the structures of united Europe that Merkel is so keen to promote.
And Greece is a problem that won't go away: its economy has shrunk by a quarter, youth unemployment is at 70 per cent, and impatience with austerity is growing.
There is growing revolt against the "troika" (the European Commission, the European Central Bank and the IMF) - especially in Greece but elsewhere as well - and doubt that its recovery plan championed by Merkel will do anything to improve people's lives.
And Merkel lacks a substantial ally. France is willing but unable, due to its own problems, and Britain is able but unwilling, as it seeks to edge away from Europe.
Merkel believes that Europe can be remade in Germany's image. But, unlike her homeland, Europe may neither need nor want its Mutti.