If you hire people this is going to happen. Most personal relationships don’t work out, why would professional relationships be any different? People grow, their needs evolve and at some point they usually move on.
When someone makes the decision to leave you’ll probably feel frustration because all those skills you taught them, all those experiences that sharpened their resume, will be walking out your door - and some other company, or client, will be reaping the benefits. Hey, you probably broke some bosses hearts along your way.
Let them go. By leaving that employee is doing you a favor. It’s better to let someone go than suffer the loss of focus that happens when they would rather be somewhere else. You don’t have the time or money that. You need people who want to work with you. And by supporting them you preserve that working relationship for another time and place.