When it comes to the fastest and most consistently growing industries in the United States, you'd have a hard time topping legal marijuana. According to the "Marijuana Business Factbook 2017" report issued by Marijuana Business Daily earlier this year, sales growth for legal weed in the U.S. is expected to hit 45% in 2018, and essentially quadruple between 2016 and 2021 to about $17 billion. This rapid growth has been a major lure for marijuana stock investors, and it's a big reason why many of these pot stocks have doubled or tripled in value recently.Image source: Getty Images.At the heart of this surge in sales is a growing acceptance of marijuana by the American public.