Was it because of time or was it personal?

Friendship; something I would like to say we all cherish. Friends are people who we can talk to, laugh with, go out with and be ourselves around. They're not just the people you'll say "hi" to in the street or who you may sit with in a particular lesson, friendship is like a unwritten contract to another person. It doesn't need to be said because it automatically should happen. You don't lie to each other, try to harm one another or do anything morally wrong. Sometimes friends can turn into best friends and you may not even realize the change, just like we don't realize the change when someone is slowly drifting out of our lives. So what I want to figure out is, is it personal or is just time that makes us drift from our best/friends. From personal experience I have learnt people don't always stick around and within a year things will dramatically change, but there are few people who I like to call true friends which WILL stick around, I haven't got many of them but the ones I do have, I completely appreciate. So is it time that causes us to drift, just slowly seeing each other less and growing into different people, no longer sharing interests and hobbies, is it uncontrollable? Or is it all in out hands, do we chose who we want to stay close with, do we make a difference to who we are friends with in years to come? My best friend happens to differ from me in many ways but then we do share dreams and interests and through the whole nine years of our friendship, we've only ever had one 'tiff' being that because of boys and hormones and we were eleven years old. Now sixteen and seventeen, I couldn't imagine my life without her. Even when everything else seems hopeless and shit, I know I've got her to keep me standing. I don't necessarily tell her the insides of my mind, but I don't feel I have to.. I can have a worry or a concern but go to her & she'll take my mind away from it and reassemble the faith in myself to just be me. This proves to me that through pure care and love a friendship can withhold it's self. But it's take two.

"A friend is one who believes in you when you have ceased to believe in yourself."