School is a funny environment. You are forced into it; you are surrounded by other kids who just happen to be there. Later, college is not that different. From this limited pool, you pick some friends, and you make some enemies. You are stuck together, and you see each other every day.
As you get older, some of your old friends remain close. You still spend free days and weekends together. Maybe even find ways to work together. And thank god you no longer have to deal with the jerks.
Then, life. It sneaks up on you, and you realize that you haven't seen your best friends in years. You got married, had kids, were busy with some urgent projects, and your weekly meet-ups or jam sessions were delayed, then postponed indefinitely. Maybe you live in a different city now. You still think of them as your close friends, and just knowing they are out there, doing their thing, is comforting.
Maybe you check in on facebook or whatever, every so often, and smile when you see them doing something fun. Maybe they hate social media, or maybe you do. Maybe you exchange a few words, but somehow that just does not do the trick. You promise to get together soon, and then there is more stuff to take care of, and more time passes.
Then you get a call. Or an email. Your friend has died. You stop, and realize you haven't seen each other in 30 years. Your best friend.