Reading C. S. Lewis is one of the wisest decisions any person can make. Recently, a passage in his book The Four Loves struck a chord. In the chapter on friendship C. S. Lewis makes the point that most of the people we call friends are actually "companions" or "allies" and that true friends are those who "see the same truth," whether that truth is ethical, philosophical or the belief that oak leaves are better and more beautiful than maple leaves (ok, that was a ridiculous example). Whatever it is, you get a connection with that person on a deeper level than you would with anyone else. The difference between "Friendship" and "Eros" (love between lovers) is that friendship is not primarily interested in the other person per se, but more interested in the truth that the other person also believes, and the fact he or she likes a certain band that you despise means nothing because you both see the beauty in a certain artist's work.
Now I know myself well enough to know that all I just wrote probably left you more confused than when you dove into the text, so I'll try to simplify what I attempted to say. When two (or more) individuals see the same truth that their companions don't, they form this pseudo-bond where they are now separated from the other companions. As friends, they are pictured side-by-side looking at a landscape, a painting or whatever truth they see, as opposed to lovers pictured looking into each other's eyes.
The main difference between lovers and friends is, lovers exclude others from their relationship. They do not welcome intrusion. Friends, if an individual meets their criteria ("do you see the same truth?") are welcomed into their circle.
Finally, after that intro, I come to my main point. Take friends A, B, and C for example. When A and B are together, A doesn't have the part of B that C draws out, when B and C are together, C doesn't have the part of B that A draws out. This is why friends welcome others who see the same truth. Many people think that if A dies, then B will have more of C than he or she had before, but the opposite is quite true. B has less of C than when A is alive. This draws out the differences of "Friendship" and "Eros" even more plainly.
Anyways, I hope you read and appreciate my rambling...all info comes from The Four Loves by C. S. Lewis and I encourage you to read it.
I also have to say, I do have two friends, B and C (they know who they are) and I can see the truth in C. S. Lewis' belief.