Well, Ygrec23, you pose a good question. It may have to do with the fact that we are 'social animals' (quoting a sociology professor) and that we crave human contact. Of course all to differing degrees depending on our personalities. But to an extent you could say we all crave love and a sense of togetherness and when we take substances that make us feel 'whole' or 'filled up' or that everything is ok, or that we're fine and successful in life (and overall sense of well-being, etc); that could substantially fill an emotional void if our social lives aren't exactly what we would like them to be. So this could easily become addictive, psychologically first as a substitute for basic human contact and love that is needed for maintainance of a healthy mental state. Then of course then becoming physically addictive as well after prolonged use. I don't know, just a thought.