Some people hate it because they have tried it and had a terrible experience with it. Others have been unable to control their usage and messed up their lives, or known someone who had that experience.
However, many people hate cannabis out of ignorance and fear. Years of continual anti-marijuana propaganda has led many to buy into government-sponsored lies. Because anti-cannabis laws in the US were passed to use as weapons against blacks and Mexicans (in the 1930s) and counterculture war protesters (in the 1960s), cannabis users were portrayed as drug-crazed outsiders, and cannabis was blamed for violence, insanity, and all sorts of antisocial behavior. The name marijuana came into common usage as part of Harry Anslinger's campaign to outlaw cannabis. "Marijuana" sounded more exotic and menacing, and prevented some lawmakers from realizing they were being asked to outlaw the stuff that was at the time being sold in drug stores for medicinal purposes.
Also, because until fairly recently it was illegal practically everywhere, the only cannabis users in the public eye were stereotypical stoner losers. Normal, responsible users kept quiet about their smoking to avoid legal, professional, or social repercussions. Consider how you would think of wine if the only wine drinkers you ever saw were street alcoholics drinking out of brown paper bags. We can hope that as state legalization spreads and the sky continues not to fall, ignorance and irrational fears regarding cannabis will fade.