I heard some pyschologist say that the reason MJ is a gateway drug is because it is actually considered just as dangerous as the harder more addictive stuff like crack, heroine. And when a young mind smokes some herb for the first couple times they start to wonder..."Hey this isnt as bad as ive been told in school and such, they are probally lieing about the other drugs too." When in fact you only need to hit the crack pipe once for a full on addiction.
Just a theory.