I can talk to my best friend about anything without feeling embarassed. I know that whatever I say will go no further if I don't want it to. I know that, even if my best friend disagrees with me, he will not judge me on my actions, but will be there to say 'never mind' when I make a wrong decision. He takes my friendship with all the baggage that entails, good or bad. Every one of these actions is reciprocated.
We share interests, backgrounds and, in our case, family (we're cousins) but I don't know how much that influences our friendship. I'm certainly not as close to the rest of my cousins, and I have fourteen of them.