In the Book of Acts, we see that Paul, Barnabas and others travelled about, frequently leaving their beloved brothers and sisters for the sake of building up the Body of Christ elsewhere. And so, Priests today are sent to different locations as well. It also avoids the cult of personality, which plagues many non-Catholic and non-Orthodox Churches. As much as we love them, and as sad as it is to see them go, it is good for them and for us to hear from a fresh voice every few years. As well, festering or chronic problems, as all parishes have, wear on the Priest.