What makes a good employee who loves his work leave the organization?

An employee who doesn't have another opportunity and leaves his organization might be as a result of retirement or forced resignation.

Stepping out from a company is mainly for the betterment of the future. Any employee for that matter if not satisfied with the benefits provided by the company may look for opportunities and step out.

Thanks / Praveen

if the manger dont want him

Simply everything can be a factor : salary , evaluation , environment around the employee , his own culture , self adapting with the team