It's hard to see great people leaving, but sometimes it's the right decision for that person. Sometimes people leave for good reasons not bad reasons. If people are getting headhunted or invited into bigger positions elsewhere, understand and appreciate that this could be because you've done a great job in building their skills and taking them to the next level. There is a great quote by Patty McCord, Former Chief Talent Officer of Netflix and author of Powerful.
"The idea of the permanent job is one of our cruellest lies - there are no permanent jobs. We do people a terrible disservice letting them think things will last forever. The biggest epiphany I had in my career was the day I decided I didn’t want to try to keep people forever anymore; instead, I wanted to make Netflix a great place to be from"