In this profession there is a never ending revolving door.
A constant wave of people inundate cities around the world to teach English. They come and go-inevitably friendships are made, and then maybe maintained. People leave impressions on each other- either good or bad. Yes, it can be sad saying goodbye, but one has to be thankful for the time spent together.
In the end, we are all one, and every person you bump into in this fish bowl is merely a reflection of yourself in some form. Every new person is another sentence in your story…perhaps a paragraph… and they help shape the way we view ourselves and the world.
We are all here for different reasons. But nevertheless, we are simply just all here.
Nomad Notions 
So true. One of the hardest/best parts of being a long term expat is getting a new cast of friends each year.