In yesterday's daf (Shabbos 15) the gemara discusses the gezera of טומאת ארץ העמים. Chazal made a gezera that חוץ לארץ has a din like a cemetary and therefore whoever goes to חוץ לארץ becomes a טמא מת.
This is a serious issue for Cohanim, how are they allowed to go/live in חוץ לארץ? After all they are prohibited from being מטמא למת and chazal gave חוץ לארץ a din of טמא מת.
The acharonim discuss what was the motivation for such a gezera? Some suggest that the motivation was ישוב ארץ ישראל, that by declaring טומאה on חוץ לארץ it would discourage people from leaving ארץ ישראל.