Why should investors like marijuana stocks? For some, the rationale might be the enormous market potential. For others, it could be simply that many marijuana stocks have soared over the last year or so.There are plenty of reasons for investors to like three marijuana stocks in particular -- GW Pharmaceuticals (NASDAQ: GWPH), Insys Therapeutics (NASDAQ: INSY), and Aphria (NASDAQOTH: APHQF).