The drug I am addicted to is alcohol. Ten years ago this week, on June 4, 1998, I had my last drink. Because the drug I am addicted to is a legal one, I was able to get help and support -- and I needed it. I had tried to give up drinking several times before, and I had failed. But because my addiction was to a legal drug, I was able to eventually find the aid I needed. If I had been hooked on an illegal drug, I might have resorted to theft to acquire it, or I might have ended up in jail.
It would be disingenuous for me to not concede that if alcohol had been illegal, I would probably not have become addicted to it. Its accessibility made it easier for me to abuse it. But again, legalization makes recovery more achievable. In the Netherlands, for example, there are fewer drug-related accidents because users are not afraid to seek help, nor are they afraid to frankly consult with doctors regarding their condition.
And while inaccesibility may have kept me from becoming an alcohol addict, and may keep some from becoming addicted to today's illegal drugs (remember, my drug of choice was an illegal drug once, too), the costs to society are too high. I live in a city where drug trafficking is a profitable and dangerous criminal enterprise. Too often in D.C., we hear about innocents being caught in drive-by shootings and drug war-related mayhem, just as drug kingpins like Al Capone wreaked havoc on society when alcohol was illegal. Legalization removes the criminal element.
The fact that I slipped into drug abuse does not give me the right to take personal choice away from you. I would not dream of saying that because I am an alcoholic, your beer and your wine should be banned. In fact, I encourage my family and friends to feel comfortable consuming alcohol in my presence. For them, safe recreational use is a pleasure, and it pleases me to see them happy. While I was never a serious marijuana user, I have consumed it responsibly and had positive experiences doing so. There is no difference, except that one is illegal now and the other was illegal then.
A nice summary of arguments for drug legalization can be found on this Wikipedia page.