One of the oddest things about friendship, to me, is the way in which a friend is able to put aside their own personal opinions about a situation that is important to another friend, and act for that person's happiness (or what that person believes will be their happiness), rather than for their own (this isn't unique to friendship, of course, but my most recent experience of it has been there, so that's how I think of it).
One experiences something of this, I suppose, when a friend wins a prize, or finishes a project. I often find myself saying, "Oh, how great!" or, "I'm really proud of you!" at these moments, and genuinely feeling those feelings: here is a situation that means nothing to me personally (I'm not finishing your article; I didn't win a prize I know you really wanted), but I'm still as pleased as if it did.
But how much more extraordinary is it when someone acts against their own beliefs or wishes simply because they believe it will make another person happy. This has happened to me, as the receiver, twice in the past two weeks, and both times I've been shocked, not by the specific act but by the fact that such an act occurs at all. People behave altruistically when they absolutely need not; they behave altruistically against their own judgement. This suggests to me that love, even the reduced love of friendship, is extremely strong.