When looking at the big picture, we're dealing with major social and economic factors. The current system makes criminals powerful, while turning innocent civilians into criminals, triggering high rates of crime, war, and poverty. People who argue against legalization cite the dangers of drugs, and while I'm not disputing the harm and danger of drugs, I'm certainly disputing how we're regulating it.
Unfortunately, we cannot eradicate supply and demand for drugs, definitely not by force, so the least we ought to do is have a system in place that minimizes the problems. Legalization will diminish the black market for Marijuana, driving the street dealers and drug cartels out of business, which will lead to subsequent decline in organized crime and gang wars, and ultimately improve the quality of life for the people.
Many people fail to realize that drug dealers and drug trafficking exists BECAUSE they're illegal; its completely fabricated (no prohibition= no trafficking and dealing). That is why opponents believe that legalization will only lead to increased drug use and crime, which is wrong.
The current system provides full protection for cartels and gangs, while providing no protection for innocent civilians. The harm done by the governing system is well and above the harm done by the drugs.
Lastly, it is fundamentally flawed to outlaw doing with your body as you please. Immorality and criminality should not be synonymous. Criminals ought to be punished by the legal system, while immorality should be corrected through education and efforts from the community.