In order to understand why people come and why they stay you need interviews. In the recruitment process, ask the prospect why they want to work for your company.
If it’s because they need a job and you’re offering one, it’s a bad fit. If they subscribe to the same values as the company, and want to be part of what your company does, you’re off to a good start. In the case of someone leaving (and especially important if it’s happening often) you need to conduct exit interviews.
Find out the real reason why they’re leaving. Is it for a better job with more responsibility? More money? Conflict of values? Lack of purpose? Poaching by competitors? A work-place bully? While job mobility is generally in the region of four years, if you are experiencing higher than normal turnover, you can’t address the problem unless you know what it is.