Deference got Moderateman to thinking of what friendship is, so MM wrote a blog on it. And the answers so far are very insightful. But thinking further on it, I wanted to add to my response to MM.
Friendship is the ability to tell your friend they are wrong when they are wrong. While they may think what they are doing is right, a true friend will get in your face and tell you why they think it is wrong to do something. If that destroys the friendship, then it was never really a 2 way friendship to begin with.
Friendship also means that when the friend stumbles, you are there to catch them or help them up. Or as DrMiler says, "sit next to them in the jail cell saying - that was a hell of a blast!". Basically it is sticking by them when they do make mistakes, and helping them correct the mistakes. Not running away at the first sign of trouble.
Friendship is also sharing in the good times. But then even acquaintances do that.
IN bad times, friendship is loaning someone the shirt off your back, and not asking when it will be returned. If it is true friendship, it will be, there is no need to ask.
God picks our relatives, but only we can pick our friends. Friendship lasts beyond the bounds of distance and time. A true friend 20 years ago is still one today, even if a continent or ocean separates you.
But most of all, friendship is not divisive. A friend will never ask you to pick sides in their fight. If you are a true friend, then you know your side. If you are a true friend to both people, then you also know enough to stay out of it until they resolve their differences. For true friends will in the end. And then both will resent you for choosing a side, whether it was theirs or the others. And that is a friendship killer.
It is often said that divorce kills friendships. Not really. But if it is a bitter divorce, friends stay away so that they do not get in the middle. And if the parties never reconcile, then there is never a chance for the old friends to reconcile either.
I guess in short, a friend does not put a burden on you. You put the burden upon yourself. They do not need to ask you, for you have already volunteered.