It's always sad when your good friend or a family made up of your good friends move away. It's not like you'll never see them again, but you also know that that person will never again have a large part in your life. Their role is reduced to "recurring character" status, and by recurring I mean you only get to see them once every year or less.
And, of course, in some cases you never will see them again. They move to whatever state or country, and then you never, ever see them again. You can keep in touch by email or social media, but then sometimes that just doesn't happen. Sometimes you forget about each other, and sometimes your friends move on with their new lives, feeling no need to keep in touch.
This isn't just the case for people moving, either. When a friend transfers to a different school, or even a different church, sometimes they're all happy with their new friends and find no need to associate themselves with the likes of you. As far as they're concerned, you're now irrelevant. All your friendship was for naught.
But then some of them put forth an effect to keep in contact for as long as it can last. They make sure not to forget your existence. Those friends are the best friends.