1. Your company is experiencing a downward spiral, losing customers, losing money, and rumors of possible closure, bankruptcy and failure prevail.
2. Your relationship with your manager is damaged beyond repair. You have sought help to mend the boss relationship but you know it is too damaged for recovery.
3. Your life situation has changed. Perhaps you have married or had a baby, and the salary and benefits no longer support your life needs. You need to move on to better opportunities to support your family.
4. Your values are at odds with the corporate culture. Your company does annual employee satisfaction surveys and you think these are a waste of time. Your company is hierarchical and you want to influence every aspect of your job.
5. You've stopped having fun and enjoying your job. No matter what changed, when you dread going to work in the morning, it's time to leave your job.
6. Your company is ethically challenged. Perhaps, the managers lie to customers about the quality of the products or the day on which product will ship. You become aware that the company is stealing information from competitors.
7. For whatever reason, you have behaved in ways that are considered improper at work. You've missed too many days of work, slacked off on the job, failed to maintain needed skills and just generally developed the reputation of a loser. That reputation, once earned, is unlikely to change, so you might as well move on, while you have the opportunity.
8. You've burned your bridges with your co-workers. Your group is not getting along in an environment that requires people to work together well. Again, at some point, the reasons don't matter; start fresh in a new job and resolve to not let this situation happen again.
9. Your stress level is so high at work that it is affecting your physical or mental health and your relationships with your friends and family. Watch for the signs of burnout and if they can't be cured, move on.
10. You are unchallenged, need more responsibility, and seek opportunities that just don't exist for you in your current organization. You've explored the current and potential options, and they are limited. It's time to move on.
Ready to quit your job? You can resign from your job in a way that reinforces your professional image and keeps current employer relationships positive.
http://humanresources.about.com/od/whenemploymentends/a/quit_job.htm