The stereotype is largely false. For some people, the body high of pot leads them to sit around lazily while high. Combine that with being high often (or constantly), and you've got a stereotypical stoner. I don't know that these same people would sit around lazily all day anyway, but the fact remains that this behaviour is based on those individuals' personalities, not the herb itself. I've always been somewhat lazy, but I'm no lazier when I smoke regularly than I am when I go without - just happier, more relaxed and generally in a better mood.