I remember that I once asked my (now) best friend, Keiran, if I was his best friend. This was anonymously on his tumblr page because I was too afraid to ask him myself. I asked him, "Who are your best friends, and how do you choose your best friends?" About a week later, I found his reply, which stated that he was best friends with about 10 girls and I wasn't included in the list and that he does not choose who his best friends are. I thought about the last part of the reply, Surely, on some subconscious or even biological level, we all pick our friends. I mean, you can choose the people who you do not want to be friends with, so why shouldn't we be able to choose our friends? I think it goes like this: we organize a set of our own attributes in our brain and juxtapose them with other people's attributes to see if they're a good match. Even if we don't realize just how meaningful their actions will be to the friendship, we know what turns us off from a friend. I suppose this is why people make a big deal about first impressions, but I'm super awkward so I was always terrible at that.