Originally posted by sango(POH):
Because sex is always going to come into the friendship.
How true is this

When Harry met Sally:
As they leave the diner, Sally defensively believes he is "coming on" to her. To carry his line of reasoning further - to get her riled up and to argue his point - Harry proposes going to bed with her. Ultimately, Harry believes that men and women cannot be friends, because sex will always interfere. [
This is a classic discussion of the film's main question: "Can a man and a woman ever be 'just friends'"?]:
Harry: What? Can't a man say a woman is attractive without it being a come-on? All right, all right. Let's just say, just for the sake of argument, that it was a come-on. What do you want me to do about it? I take it back, OK? I take it back.
Sally: You can't take it back.
Harry: Why not?
Sally: Because it's already out there.
Harry: Oh jeez. What are we supposed to do? Call the cops? It's already out there!
Sally: Just let it lie, OK?
Harry: Great! Let it lie. That's my policy. (They get into the car.) That's what I always say. Let it lie. Want to spend the night in a motel? (She glares at him.) You see what I did? I didn't let it lie.
Sally: Harry -
Harry: I said I would and I didn't...I went the other way...What?
Sally: We are just going to be friends, OK?
Harry: Great, friends. It's the best thing...You realize, of course, that we can never be friends.
Sally: Why not?
Harry: What I'm saying is - and this is not a come-on in any way, shape, or form - is that men and women can't be friends, because the sex part always gets in the way.
Sally: That's not true. I have a number of men friends and there is no sex involved.
Harry: No, you don't.
Sally: Yes, I do.
Harry: No, you don't.
Sally: Yes, I do.
Harry: You only think you do.
Sally: You're saying I'm having sex with these men without my knowledge?
Harry: No, what I'm saying is they all want to have sex with you.
Sally: They do not.
Harry: Do too.
Sally: They do not.
Harry: Do too.
Sally: How do you know?
Harry: Because no man can be friends with a woman that he finds attractive. He always wants to have sex with her.
Sally: So you're saying that a man can be friends with a woman he finds unattractive.
Harry: No, you pretty much want to nail them, too.
Sally: What if they don't want to have sex with you?
Harry: Doesn't matter, because the sex thing is already out there, so the friendship is ultimately doomed, and that is the end of the story.
Sally: Well, I guess we're not gonna be friends, then.
Harry: Guess not.
Sally: That's too bad. You were the only person that I knew in New York.