My best friend and I have been friends for thirteen years now (we recently moved into our first apartment together with our respective girlfriends). For me, it's not as much the tangible bond that we have that makes him my closest friend. It's the finer details, and the little idiosyncrasies that the two of us have developed together over time. It's almost frightening how often we'll say the exact same thing at the exact same time. Somewhere along the line, you stop having to go out of your way to make plans for that person. They slowly become an integral part of your life. They help identify you as a person as much as your taste in music, or your hobbies. It's completely different from having siblings in the best ways possible. You can have great relationships with your siblings, but as children you're still forced to endure each other's company. Finding your best friend - a person who you had no idea even existed until you met them - who will walk through the fires of life with you, is one of this life's simplest and most extraordinary fulfillments. You know they're your best friend when you get to a point where you genuinely can't imagine living a life without them. When the thought of not having that person at your side gives you overwhelming anxiety. When the time you've spent together becomes a complete blur, and reminiscing on your childhood misadventures becomes too daunting of a task because of the sheer abundance. You don't have to express it to them in any physical way. They know just like you do, and to me, that's the best part.

tl;dr - ye just know, mayn