Why employees leave organizations?

 I was in a situation of finding the reason for this argument. This is like two well sharpen sides of a blade. And also pretty much argument-able point that we come across in our life.

According to my philosophy this has many factors to consider. Some may say that the person who are leaving is a person who is incapable of performing the job. But also we can argue that the person who is leaving the organization is a person who is capable of finding another suitable job in the today's competitive job market. See, its pretty fair enough reason to argue with.

I have personally seen organizations where Human Resource Department is very much proud saying that they have employees who are working in the same organization for more than 10 years, 15 years and so on. But can't one can argue that they are the incapable persons who are in the same place for years because of unable to find another place to work. Suddenly you might think that, that is because the company treats the employees well. But really can that be the only reason for this? May be not, isn't it.