Acceptable reasons for leaving a job include a lack of advancement, extreme stress, not being able to use existing skills or education, and a lack of interest. An individual might also choose to leave his job because he isn't certain how long his job or the company may be viable.
An individual who is under extreme stress or suffering from health issues because of his job would be justified in quitting. A majority of individuals also want to know there are opportunities to advance in a company and learn more about the industry. If not, an individual may decide to look elsewhere for a job in which he can earn more in a higher position.
An employee might also desire to obtain specialized training and learn valuable career skills while working with a company. Even if there aren't management positions, an employee may remain with a company if there is useful information and skills that may eventually help further his career. Before an employee decides to leave a position because of a lack of learning opportunities, it's best that he bring the issue up with his manager in order to ensure he has explored all of his options.
Boredom is another good reason to leave a job. The employee could be doing the company a favor by quitting if he is no longer passionate about his position since it could negatively influence his job performance.