The Concept of "Best Friend"

This morning I saw a post on BlogHer called "I've Never Had a Best Friend" by all.things.fadra. She wrote:

I used to get offended when I would meet people, especially people I really liked, and they would tell me about their "best friend back in Michigan" or the woman they've been friends with since the second grade.

I took it to mean: Hi. Nice to meet you. We can be friends but not that good of friends because that position has already been filled. And perhaps I stayed guarded in the...