what is a friend?

I have been asking myself this question a lot lately. I am not sure if I am any where closer to finding an answer. I know what a friend isn't, some of the time. I know what a friend can be. I know what friends have been in the past. But to make a definition of what a friend is, seems to be nearly impossible.
It seems that friendships, like anything good in life, are not always easy. They wax and wane like the moon. It seems to me, in my experience that in order to gain a good friendship you have to be good with yourself first. You have to know what you want out of a friendship and what you are willing to give up and put up with. You have to be willing to put the time in to a friendship. If you don't spend time you will never know one another and you will never be comfortable.
However, as I sit here, rereading what I have typed it occurs to me that I may very well be full of shit. Do real friendships feel like work? Does it seem like a laborious task that you add to your to do list? Probably not. Perhaps friendships just.... are.
Like I said, I am no closer to answering this question than I was when I first pondered it.