The employee and employer relationship can be very delicate. It typically starts off well and both are eager to impress each other, whether to provide a good service or provide a good working environment. The reality is that even under the best intentions and best circumstances this relationship can become strained. I am sure that everyone can think of a few situations where this has become the case.
What I am alluding to here is that many employees have trouble identifying when they've been at a job too long, have reached their potential, and instead of growing, they become disgruntled employees over time. I am sure that everyone has been guilty of this at least once in their career. This of course, applies to some industries and environments more than others but for the sake of this discussion, we'll keep it generic.
The employer can be as guilty as well. They may develop a loyal connection to these long time employees who have probably helped through some tough and good times. Though at some point they are unable to provide new opportunities for growth and advancement.
This is a very unfair system that starts to grow on both parts. The employer recognizing the commitment and history of these employees may tend to promote due to loyalty and not on merit. The result is employees who are eventually promoted to a level of incompetency. This can lead to an employees dissatisfaction at a job.
I'm not implying that anyone should be fired in a case like this, but it is important that both the employee and employer are aware of this. In an ideal world an employee can address this with their employer and if an employee is ready to make a change, to support it and help them with new opportunities. The worst thing an employer can do is try and entice them to stay by simply offering them more money. It's a very short term fix only.
Ultimately, don't stay at a job out of comfort and fear of change. It is much better to leave when things are good and you feel that you've accomplished something. Having been on both sides of the employment relationship, I believe very few employees take full benefit of the opportunities that an employer can offer, even outside of their immediate firm.
For employers, I believe that a primary goal is to provide them with an opportunity to do a good job, and build their skills which will help them land and succeed in their next job.