I think I've shared this one before but I had a brief relationship with a girl many years ago who was far, far more interested in me than I was in her. I was, and still am, quite an independent person and don't like to be tied down - but she was the opposite and making all the effort. I suppose I was never firm enough to tell her I wasn't interested and it came to an end on a night out where I'd left her with her Mate and spent more time talking to friends than I did talking to her. She dragged me to the outside area of the Pub to give me an ultimatum (i.e. tell me you love me or fuck off) so I tried to say it wasn't going to work out but did so without any tact. She promptly burst in to tears and all I could do was laugh in her face which I put down to being deeply embarrassed at the entire thing.
I'll never forget laughing in the face of a girl who was crying because of me. Never.