Most have their own definition of a friend - generally speaking, that definition is usually deeply ingrained with what they "expect" the friend to do for them, which is often incompatible with what someone else can give. Not talking just materialistically, although that could be part of the equation, I'm saying things like listening are action- oriented on the part of person just as certain behaviors are doing. Therefore, I think friend is a verb that has mistaken for unknown for far too long. This is not a grammar lesson however, the semantics of friend as being that which he or she actively does is very different from friend as a thing or object.
Friends are people; people are unpredictable. Time it's self is ambiguous. Knowing this then we should contextualize the word friend as it relates to a specific time and place - a moment, in essence. Friendship is momentary, some moments last longer than others, but they are still just moments, just as our life, itself, is a series of moments.
Don't have friends, BE ONE.