I don't think friendship is a state as much as a process, which is related to how much you share (and keep sharing) outside of your common interests. Also, how much you can turn to each other for help plays a part.

Two quotes come to mind. They're jokes, but still sound apt to me.

"Friends help you move. True friends help you move bodies."

"Friends bail you out of jail. Real friends are there with you and go: Whee! That was fun!"