Challenges = Tenure
But one reason that a company has control of is that people just get bored doing the same thing in the same place with the same people for the same amount of money. There are just no challenges or thrills left at the old job any more, so they start looking around for a greener pasture.
Robert Wayman was the CFO of Hewlett-Packard for 23 years and a 37 year veteran of the company. He was ready to retire when things got a little dicey and the company needed help cleaning up the mess associated with board-directed detectives. Why did he postpone his retirement? New challenges. "How long you stay with a company depends on how many learning opportunities are available," he said (CFO magazine, April 2007, http://www.cfo.com/article.cfm/8885490?f=search).
If you have ever just stuck it out at a company where you were totally bored, but just could not quite leave, then you know what a living hell that can be. Eventually you even become ashamed of yourself for not having the initiative or courage to walk out.
Pursue the challenges and let the rest take care of itself.