There comes a time in everyone's life when, ya know, you're just pretty much over it. You don't have time for people who don't actually care about you anymore. You've grown up and realized you don't have to be friends with someone just because you've known them for ten years. Sure, there's some people that you've known since birth that you can rely on, but knowing someone for a long time doesn't mean much nowadays.
For example, I became friends with someone in sixth grade. We did everything together; we were the best of friends. However, now I have no idea what she's doing with her life and we haven't spoken in almost two years.
On the other hand, my best friend since pre-school and I are still going strong.
But like I said, it's not who you've known the longest. It's who's there for you when you need a shoulder to cry on. It's who's there for you when you just failed three tests and need someone to eat junk food and watch movies with, or who's there for you when you need help making the hardest decisions in life.
It's the people who defend your name when you're not around, stand up for you when you can't stand up for yourself, and shine some light on your day when you're having one of the darkest.
They're the people who you can call up after not talking to them in three months and not feel uncomfortable saying, "Hey, I'm coming to see you!"
You can show up at their house (or dorm) unnanounced and instead of telling you they're busy, they welcome you with open arms.
You know everyone they hate (and probably hate them, too), know everyone they like, have dated, and pretty much every other detail of their life.
And hopefully, if they're really awesome, you can discuss all of your bodily functions with them and they won't call you repulsive.