I have a best friend, and she will ALWAYS be my best friend. I think it takes a lot to become somebody's best friend. You have to go through some pretty tough stuff and bond in ways that you can't with most people. There is always a deeper connection than with other people. That's why we can finish each other's sentences or understand each other when what we're saying is a big jumble of words.
It's helpful to have a best friend because you know that they'll always be there for you, no matter what. If you fight, though, that really hurts. When you trust someone that much and give them so many pieces of yourself, losing them would be like losing yourself, too.