This is from personal experience from being a part of the Human resources team in a software multinational and observing why friends quit their jobs.

There is no definitive reason or one clear cut answer - as every individual is unique and will have his/her reasons. But I believe that one's choice is usually compelled by not a single reason but as a consequence of multiple reasons. Thier magnitude of influence being key!

1. Passion- People love to do the work they are passionate about. When one starts loosing interest and finding purpose and meaning in what they do, there is a tendency to look out. This is usually the case when one's perception of their work's impact is distant to their effectiveness and relative importance
2. The Boss and co workers- Every self respecting individual with certain credibility and education would find pride in his/her work. When not suitably motivated or when there is lack of communication from their boss/reporting manager there is a sense of isolation. lack of clarity and coworkers not keeping a team member in the loop are clear demotivatiors
3. Lack of vision/career path - Most individuals have set goals and needs from a career perspective. When their aspirations are not in sync with those that the organisation plans for them, they will feel left out. Some teams do not charter individual training needs as part of their growth, then there will be discontent in the mind of the employee
4. Socio economic reasons - Peer pressure is a deep influence especially in the earlier stages of one's career. If one's friends went to the same school or are part of the same industry, there tends to be comparison and percieved notion of dissatisfaction, because, lets face it! most people tend to crib about their jobs and the extent of agony each is facing may compel one to quit their positions. Economy doing badly affects the performance of the company that one belongs to, if there is a lot of discussion on the health of a company that one belongs to people may panic and quit thier jobs due to the uncertainty
5. Personal resons - This happens only when someone is relocating, has health problems, plans to move into a different industry

Considering the above reasons and having taken exit interviews, I feel the most visible signs to change a job are:

• If you feel like you have a prolonged state of monday morning blues where you dont feel like getting to work each day
• If you feel you are walking in a blind alley where each day you are told to do things without the visible outcome being elucidated clearly
• If you are stressed out while having ocnversations with your boss or colleague and cant see eye to eye with most of them
• If you are not rewarded suitably and if you think you are paid lesser than industry standards and are not motivated enough
• If you feel that the company/organisation is not all that meets the eye and divergent in its values to your own
• If you have risk of health hazards - its not worth it in the long run cause no matter how much you earn it will not help you overcome/justify the suffering of a health condition

However, if the economy is doing as badly as it is and when everyone are loosing jobs, being patient, rational and objective in defining your short term career needs may help you avoid quitting a job on impulse!!

Back home in India, we have a quote where we define the basics of existentialism as roti, kapda aur makaan which translates to bread, clothes and shelter. If these primary necessities cannot be met if in a state of unemployment, my sincere request would be to hold on to your job as it will be a means of sustenance.