Often people can be unhappy about management or office changes, or have become frustrated that their training needs are being ignored. They simply feel frustrated they haven't been promoted. Alternatively, they might simply fancy a change or have been given a once-in-a-lifetime chance to further their career, start their own business or even go travelling.
Yet in such a people-business as media, agencies and media owners are trying hard to convince their best staff to stay by ensuring they are content in their work.
Here, Media Week looks at the main areas companies must focus on to attract the best and to make sure they keep them.
WORKING CONDITIONS AND COMPANY CULTURE
The fact MediaCom even has a person with the job title "director of freshness" demonstrates how important the working environment and company culture is in media.
Media people need to be stimulated and if they become stale the agency and its clients can suffer.
"You need to make the workplace exciting, interesting and challenging to keep staff interested and therefore happy," says Matthew Mee, MediaCom's freshness guru.
"We hold activities around the office, such as tasting days with food from around the world, or invite an expert in to show an account team how to throw pots."
Carat UK managing director Neil Jones says media people can burn out at an early age in what is an intense business at times, so finding ways to avoid this is crucial.
"As a global company, we offer to transfer people to offices around the world including Australia and the US," says Jones. "We also provide people with free food and drink in a chill-out room and have spent a lot of money refurbishing the building. We're trying to make the offices even more open plan, so people are aware of what others in the business are doing and the pressures they are under."
The location of a media company, whether in central London or a city like Manchester, can certainly have an impact on whether people stay or join. The presence of an internal, subsidised bar can also be crucial in shaping a company's culture and boosting the staff's feelgood factor - as it is usually the centre point for celebrations whenever new business is won.
Sales staff at MSN benefit from the financial clout of Microsoft when it comes to creating an attractive working environment. There is a lake and nature walk at its Reading HQ, which staff are encouraged to stroll around. Its canteen openly promotes healthy eating and there is a Wellbeing Centre where staff can get advice and even treatment.
It may be a well-worn cliche, but the majority of media companies do work hard and play hard.
Most will host summer and Christmas parties or take staff away for a few days to bond the company's workforce together and keep spirits high.
Walker Media took about 60 staff to Prague for two nights last year, having previously organised trips to Madrid and Barcelona.
"You can measure the success of these trips by the camaraderie and banter that goes on before, during and after an event," says Walker Media partner Phil Georgiadis.
"The key thing is people appreciate being away from the usual daily grind of office life."
Simon Trigg, managing director of online advertising specialist DGM, says it is important people learn to celebrate and commiserate as a team.
"You can always sense if people are not happy. There may have been a couple of disappointments on pitches, so getting everyone together can put things into context and help people realise they cannot win every time and they have to take the rough with the smooth," he says.
"Also, the people responsible for organising big social events can raise their own profile within the company, because they are involved in something that everyone else is really looking forward to."
OMD UK's deputy managing director, Jonathan Allan, agrees that a strong social side of any company is vital to motivating and retaining good people.
"It is important we all respect and care about other people's roles and social events help people to understand what other staff do," he says.
CAREER PATH AND TRAINING
One of the main reasons people leave an agency or media owner is because they do not feel appreciated.
It is vital anyone who joins a company can see a career path ahead of them and that their training needs are addressed.
Any agency or media owner worth its salt will have internal and external sessions planned over a new entrant's first year, and ideally provide them with a mentor.
"Identifying a person's career potential and judging how fast they should progress is vital," says I-Level's HR director, Kate Douglas.
"A company must get its succession planning strategy right, so it knows who is ready at what point. Because if it gets this wrong it can lose good people to the competition."
Former Carat new business director Jenny Biggam, who along with fellow directors Colin Mills and Mark Jarvis set up The7Stars last autumn, says: "Media agencies and media owners are full of young ambitious people and if they feel their opportunities to move up the ladder at one company are limited they will move on.
"The only way to solve this would be to create different hubs with individual management structures servicing clients. But this would be less efficient, so most media agencies will remain a massive pyramid in terms of staff structure."
It is essential managers are aware of an individual's training needs. At broadcaster Five, head of HR Julia Dell says the company tries to provide all staff with a portfolio of skills. "The more skills people have the better their performance," she says. "Five is 10 years old and about 30% of the staff have been here all that time. So they must be quite happy."
SO CAN YOU - AND SHOULD YOU TRY TO - STOP A GOOD PERSON LEAVING?
It is certainly possible to change a person's mind about leaving if a company makes the effort to address their concerns. This is not necessarily about paying them more, but about assessing whether their needs could be met without them having to change employers.
Could they be moved into a department where they really want to work, maybe from the press team into TV or from classified sales to display? It may be the case that a talented person has been promoted out of a role they really enjoyed and are unhappy in their new position. They would happily stay if they could be redeployed in a way that meant they did not lose out financially.
Another way to retain people is to switch the clients they work with. There may be a personality clash with someone on the client team or ethical reasons why they do not want to work with a particular brand.
Money can be a red herring when someone wants to leave, but is it really worth trying to keep someone who is obviously unhappy?
"Organisations must be careful because if they help one person they must do it for all, so there must not be a knee-jerk reaction," says Paul Farrer, chief executive of recruitment agency PFJ. "After all, many people who are offered new jobs in these circumstances still end up leaving within six months anyway."