Marijuana should not be legal because it alters your perception, meaning that you could get into a wreck or do something you won't remember. You could hurt someones family. That would be something you'd have to live with forever. Plus that jail time that you would more than likely have to serve.
Making marijuana legal would also allow people with illnesses to treat themselves in case the doctor doesn't believe that it will help, but that person may believe that it helps with their illness. Some Doctors may believe that it will help their patients but not everyone is going to go to that doctor.
If we make marijuana legal, it will lower the organized crime rate. If people could get marijuana for 1) easier without getting in trouble, they're going obviously go with that, 2) they are spending more money but they wouldn't be getting in trouble.