Friends give each other complete freedom to be themselves. When you've got a friend that close, that knitted to your own soul, you don't have to explain why you do what you do. You just do it, and your friend understands.
When your heart is broken, you can bleed all over a friend like this, and he won't be offended. He won't confront you in your misery. When your good friend is hurting, let him hurt. If she feels like weeping, let her weep. If a good friend needs to complain, listen. An intimate friend doesn't bail; he's right there with you. You can be yourself, no matter what that self looks like. True friends are a constant source of encouragement.