If you are extremely calculative and overly analytical and if you start to guess (and assume) the exact reasons why your friends are actually friends with you, you would realize that most of them have ulterior motives to maintain your friendship. Or, if you have some good friends, you may realize that those people are not friends with you for any particular reason(s). There is a strange sense of satisfaction that is derived from the fact that reasons for those friendships cannot be trivially articulated. This of course is a mere difficulty in expression probably due to lack of development of enough words for all possible emotions in literature and cannot be attributed to the "quality" of the friendship. But it can be safely argued that best friends usually belong to this inexplicable category.
We have a vague notion of differentiating friends from best friends using the limits of language and expression abilities. But this theory, even assuming is 100% true is not worth a penny. There is one more simple succinct theory on friendship (rather acquaintanceship) based on communication that will be worth a life times learning - which most people already know, but face great difficulty implementing. "If you need friends, you need to listen!" Seems trivial and easy but its not (at least to me).
Today I had to make a simple conversation asking for a small (probably useless) gift for a friend from a stranger. I was bold enough to initiate the conversation ( I am very happy about it!) but was so embarrassed in the middle of the conversation that I didn't actually realize the fact that the female said something synonymous to "yes- u can have it". In essence , I asked for something from a stranger for which she said yes (in spite of my record poor communication skills) and I didn't register that yes (probably assuming pessimistically a NO!) and walked away like(?) a total fool.