Maybe they felt unappreciated, or overworked. You employ them, you don't own them. Why is the modern workplace starting to sound more like slavery than a job?
So one cannot feel frustrated if an employee leaves, or you're running a slave shop?
(I'm aware I'm exaggerating your comment. My point is OP can feel frustrated/angry if an employee leaves. It doesn't mean he's exploiting his employees).
I'm not suggesting that it is unreasonable to feel disappointed when an employee leaves. However if a "star employee" leaves... usually it means that the employee didn't feel like a star employee. You're dealing with other individuals who have different goals in life. Don't expect them to be exactly like you or be completely dependent upon you.
Maybe they felt unappreciated, or overworked. You employ them, you don't own them. Why is the modern workplace starting to sound more like slavery than a job?