Employees quit for a smattering of reasons from not enough personal time to conflicts with other coworkers. 
A question we often get is: Should we rehire an employee that previously quit?
We always look at two sides of the coin when answering this question for our clients.
- Why did they originally quit?
The reasons for why an employee previously quit is very important. Unless those areas were corrected during the period in which they were gone or can be addressed before they are rehired, the employee is likely to leave again. A great example is an employee who stated that they did not fit the culture in your company. This is a difficult area because if the culture has remained constant since they left, then there is a high probability they will leave again.
However, if they left due to personal reasons; plans fell through or took a different route (new addition to the family, moving, etc.) then a transition back may make sense.
- How did they perform the first time around?
Past performance will tell you a great deal on IF you want to rehire a person, if you feel their answer to the first question was reasonable. If you felt this person was mediocre or relatively neutral in their performance, you should continue your interviews and view them as you would any other candidate. Entertaining them is fine and may be attractive because they are coming trained and knowledgeable of your organization and product but you do not want to pass up on an outstanding new employee simply because they will require training time.
Start by asking the previous employee why they decided to reapply for their position and what they believe has changed in them and/or the organization to make them reconsider coming back. Your goal should always be to hire the best employee for the position. Whether it is a former employee or a new prospect. The best candidate wins!