how much of a friend is how well you know them? how often you hang out with them? how long you hang out when you do? how much you talk with them? what you talk about? how much of a friend is how long you've known them? the history you've shared? mutual interests? how much of a friend is simply some unknown chemistry/pheromones/yogsoggoth/fnord?
what do you think of when someone asks you if "you and soandso are friends"? try rating the items in the previous paragraph in order. or on a scale of 1 to 10. does the order vary depend on the individual friend/friendship? if so, are there different types of friendships? different things you use the word friend to mean?
bonus question: where does love fit into all of this?