"...They can't be friends...unless both of them are involved with other people. Then they can. This is an amendment to the earlier rule. If the two people are in relationships, the pressure of possible involvement is lifted. That doesn't work either. Because what happens then is the person you're involved with can't understand why you need to be friends with the person you're just friends with, like it means something is missing from the relationship and wanted to go outside to get it. Then when you say, 'No, no, no, no, it's not true, nothing is missing from the relationship,' the person you're involved with then accuses you of being secretly attracted to the person you're just friends with, which you probably are - I mean, come on, who the hell are we kidding, let's face it - which brings us back to the earlier rule before the amendment, which is men and women can't be friends."

Harry, en When Harry met Sally