I understand this question a little different from the dimension @cryptoandcoffee answered it from. I understand the question to mean what can cause one to fear resigning from a company and I am going to respond to it from that angle.
Fear is one feeling that can cripple one and stop progress. Having worked in a company for a long time, it is normal to have a fear of resigning from that company due to so many reasons and some of those reasons is what I shall be enumerating below.
1. Sense of stability and continuity. This is one reason why people fear to resign from their jobs. They are used to the same routine. They wake up same time, get to work at the same time, do the same job requirements comfortably and they have been doing this for years that they feel comfortable. They know they need to aspire higher, to earn more and make a better living but they are too stable to want to take any risk.
2. Relationships and reputation. You have been working for years there, your colleagues are more than just colleagues, they are family now. You spend weekends together and some are even God parents to your kids and then even though the working condition of the company is no longer conducive, you do not want to leave because it will feel like you are betraying your colleagues and the company.
3. You've lost track of the Labour market requirements. You have stopped writing applications for a long while now and you don't know what your skill is worth in the Labour market anymore. You are afraid to find out. What if they say you ain't qualifier, what if your age is above the age limit required now?
4. Bills to pay. Whether you resign or not, the bills won't stop coming. There are electricity bills, gas bills, house rent, cable bills etc to pay. If you resign now, how do you manage to keep paying this bill? If you have a wife and kids this makes it more difficult to take a step towards resignation.
5. Lack of finance to start up. This is for those who want to go into entrepreneurship. You want to be your own boss but you lack the finance to start up any business. Quitting your job won't give you the needed finance and so the tendency to keep working and saving some portion from salary until you are able to save enough to start is there. Again, this may take a long time depending on how much you earn.
6. No new job waiting for you. You don't want to quit your job and go sit at home doing nothing. What will people say?
These are some of the reasons shy people fear resigning from a company. To quit take a courage and putting your long term and short term goal into consideration.
When someone resigns from a company the others that are left behind always ask why. the work colleagues wonder if they are getting paid correctly that someone has resigned to go somewhere else.
It is an obvious reaction from your work colleagues. is the company in trouble, do you know something we don't? It creates fears when normally there are none. people are paranoid and don't understand that sometimes moving to further your career is healthy and good.
The only time one should worry is when there are loads of resignation as then something is wrong. It could mean a whole host of things.