Some friends tend to come with an expiration date. You know exactly what I mean too. There are some people you're friends with for a certain period of time, and then they disappear. You never talk to them or hear from them again. They just vanish. Their loss.
Then there are friends that are drifters. They drift in and out of your life here and there. Some of them are really good about being around when you need them, and some come around when you really wish they wouldn't. What can you do? Sometimes you're lucky, sometimes you're not.
And then there are those friends that are... a constant. Though you may not see them as often as you like, they're always connected to you somehow. You never go too long without some kind of communication. And even if you don't get to see them regularly, when you do, it's like no time has passed at all. Those are the ones you should pay extra attention to (not that all of your friends shouldn't be appreciated). Those are the ones that will always be there for you, no matter what happens, no matter how much time passes, and no matter what you do.
Make sure you pay attention.