The government - not your party - has the machine, the power, to set the agenda. Resources are scarce, expertise often dependent on goodwill.
Even if, like Jeremy Corbyn you have just achieved a thumping victory, controlling events is impossible and pulling together a party where hardly any of your colleagues agree with you makes that job harder still.
Yes, the scale of his success means he is safe from challengers for now. But having turned down any major public appearances or chances to tell the public more of his plans today, he has spent the afternoon holed up in the Palace of Westminster.
His ambition to unite the party is characterised by his first objective, putting together a shadow cabinet that is not just packed with left wingers.
But although Corbyn has been discussing the team with one of his disappointed rivals, Andy Burnham, and other senior figures like former cabinet minister Lord Falconer, there are already signs of trouble.
There is anxiety over his potential appointment of his close friend and campaign manager, John McDonnell. One senior MP told me that would be a "declaration of war".
Others are pressing for the job to go to a woman, most likely Angela Eagle.
Although many former shadow ministers have already walked away, standby for the emergence of the Make it Work Brigade - those in the middle of the party who will take a job in Corbyn's team to stand their ground.
For Labour MPs this has been a mind-bending 24 hours - it will take a long time to acclimatise to having someone they consider to be on the fringes, in charge.
But after yesterday, together with his massed ranks of supporters, in charge he most certainly is.

PC Neil Doyle, 36, died after he and two colleagues were attacked near the Aloha Club, in Colquitt Street, on a Christmas night out on 19 December.
Three men have been accused of murdering Mr Doyle and are due to face trial on 22 June.
The match was played between Ormskirk Rugby Club and Merseyside Police's rugby team.
They were due to play the fourth Green & Blue game, named after the respective colours of the clubs, on 20 December but the match was postponed following Mr Doyle's death.
A post-mortem examination found the cause of Mr Doyle's death was bleeding around the brain.
Christopher Spendlove, 30, and Andrew Taylor, 28, pleaded not guilty to murdering the officer and wounding his two colleagues when they appeared in court on 20 March.
Timmy Donovan, 30, formerly of Huyton, has been remanded in custody after he was extradited following his arrest in Germany, in January. He is due to enter his pleas on 8 June.
Nearly Â£30,000 has been raised for a memorial fund in PC Doyle's name, with donations going towards Merseyside Police Federation Charitable Trust.
PC Doyle was based at Eaton Road station in West Derby, Liverpool.

Kent finished second in Division Two last season and were not given a spot in the top flight after Durham were relegated because of financial issues.
Gillespie, who joined Kent as cover for Allan Donald in March, led Yorkshire to promotion from Division Two in 2012.
"For this whole squad, the world is their oyster," he told BBC Radio Kent.
"Kent have shown that they can do it and they can challenge to play in the first division.
"They play against first division sides in white-ball cricket all the time so the lads know they can do it, it's just a couple of things falling into place."
There is a long list of very good reasons why being leader of the opposition is known as the hardest job in British politics.
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A rugby match has been held in honour of an off-duty police officer killed in Liverpool last year.
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Kent interim assistant coach Jason Gillespie says they can challenge for promotion to Division One of the County Championship this season.