When you have good people around you, working for you, you want to keep them forever. But one of the most important roles of leaders is to mentor others, and with it, helping them to reach new heights that will some day take them somewhere else.
In this process, identifying the right people is just the beginning, but then considerable effort is placed into helping budding professionals move into roles with more responsibility. Watching them evolve is a lot of fun, getting the confidence they need as they move along.
Eventually comes the time when a mentee finds a better opportunity, where they can live up to their full potential. In part, the mentor is responsible for this, and it is a success. But it is also a sad day, because you will not keep them forever. There is a word for that: Bittersweet.