Why is Harry...

I think that by the way Sally acted afterwards, like a love sick puppy, Harry knew the relationship would become doomed. He didn't feel THAT way about her until before the last scene in the movie, where he realised he actually was in love with her. Remember he said before about how men always think about how long they have to stay and hug a woman after sex so he's probably thinking he didn't expect to be in this situation with his good female friend.