I think it's about ones own perception and willingness to...'go further'. I mean, if you smoke herb and are comfortable with the high, you may not have a desire to do or try anything else. Be it stories you've heard, people you've seen, or just lack of interest. On the other hand, if you smoke, enjoy the high, and wonder if there are other or 'better' highs you can achieve, then you might have a higher chance in branching out and trying other drugs.