The thin line between acquaintance and friend

I noticed something the other day while talking with one of my best friends. There is a line between a person being an acquaintance friend and a person being a true or best friend.

I don't quite know how to describe it. There's a transition with some people, but when there's not, two people might keep on trying to transition until one of them says something and then moves on.

I've had that happen in my life a few years ago. One of my friends emailed me and said something to the effect that our friendship wasn't working out, so goodbye. Well, at the time, I was just confused and thought to myself, "I still consider you my friend" and I tried to convey that in an email. But to no avail.

NOW, with the understanding of that 'fine line' I begin to see that there was some barrier to one of us (or both of us) being able to cross that line. What was it? Different culture? Different life goals? Different what? What WAS the difference between that person and the people I consider my best friends?

I have no ready answer. There IS a line, though, that once crossed, changes a friendship into a best-friendship. It's a 'familial' experience. Once the line is crossed that person becomes more like a trusted sister (or brother). Trust defines part of that thin line, I think. But what else? What changes when someone becomes a best friend?