Why do people say "I Love You" and knowingly they do not mean it?

How many people have been in this situation? Was it just a drug or alcohol-induced frenzy that made you tell someone this and not mean it? Or, is it only when players are playing and those three words are as commonplace to say to them as thank you? I ask this question, after watching the music video of Wham and their hit song Last Christmas. It was very obvious that she loved him, but he was there only for a one night score. Question: how can a person tell another person they love them, do not mean it, and live with themselves? I never was in this situation, how about you?