Your business wouldn't exist without your best employees. Unfortunately, many employers feel blindsided when they get a two-week notice from a top employee. They shouldn't. There are plenty of red flags, and nurturing employee loyalty is something that takes constant effort.

There are countless reasons why employees quit, and not all of them can be addressed. However, the five major reasons employees quit can be wholly preventable if the employer takes notice--and action.

A subpar reward system is the number one reason employees quit.

If you reward your employees, it instills loyalty.

It doesn't have to be financial, although that's a great incentive. Corporate recognition on all levels can do a tremendous lot, and be sure to focus on both internal and external recognition. Employer paid mini vacations, taking over a project, promotions, and donating to charities in their honor are also great incentives.

However, bear in mind that rewards are useless without communication. Find out what your employees really want and prioritize those rewards.

The second most common reason people quit? Their managers.

Many times, an employee is generally happy with the company, but maybe not a particular manager.

There can be natural tension between managers and their employees, but sometimes it's just not a good match. Something as simple as changing managers or teaching your managers how to communicate better can keep great employees on board. Management isn't for everyone--in fact it's a very tough job. Are you sure your managers are management material or would they thrive better elsewhere?

The third most common cause of a notice is hiring and promotion issues.

If an employee keeps seeing their colleagues getting rewarded for brown-nosing, that breeds resentment.

Some employees may be "manager's pet," which reads as a direct insult to those who are truly giving their all. Keep an eagle eye on these issues, and be sure not to penalize the "pet" for a manager's mistake. Instead, develop and stick to fair ways of rewarding and promoting workers.

The fourth cause of calling it quits is the employee having too much work.

We live in an era where multi-tasking is the be-all, end-all skill--even though research has shown virtually nobody can do it!

If you keep demanding more, better and faster from your workers, they'll reach burnout quickly. There's a cap to how much someone can do, and success ebbs and flows. Still, some employees stick by their work doggedly until it suffers so badly they get fired. Either way, the company loses.

Finally, There is a fifth reason to consider, as well, Layoff Rounds.

When you start laying off the workforce and downsizing to look good for a funding round or an acquisition, make sure you are transparent with your remaining employees.

One sniff of the company looking like the Game of Thrones' Red Wedding, and people will start leaving in droves. Not only that, but they will say rude things about you on Glassdoor.

Going into 2016, it's time to identify your top workers, treat them right, and work to keep them on board. It's a lot more time consuming and expensive to train a new employee rather than preserve an existing one. Focus on what you already have in your employees and treat them with the golden rule. It'll ensure this year is successful, and remember that happy employees are productive employees.

Are there other reasons that employees may leave a job? Feel free to share your thoughts below in the comments!