There are plenty of arguments for it. I lay out the ones that I believe in.
1. Self ownership. The basic concept that I own my body and no one, particularly the government, has the right to tell me what to do or not do with it. The government has no business telling me to lay off fatty foods, get my exercise, or get eight hours of sleep each night. Nor does it have the right to tell if I can or cannot injest marijuana.
2. Consistency. Alcohol, as we all know, is legal. Yet all of the bad things marijuana is blamed for or associated can be equally applied to alcohol, except in much much larger numbers. A lot of pro pot advocates argue that pot is not nearly as bad for the individual or society as alcohol. I won't even go that far. I'll just state that pot is certainly not worse than alcohol. And if pot is no worse than alcohol, why should one be illegal and the other legal?
3. Black markets are bad. Despite our laws, demand still exist for pot and it always will. Which means our prohibition of marijuana creates an illegal black market. What are the affects of this black market? Marajuana is unregulated. You often here stories of it being laced with other dangerous substances without the buyer's knowledge. That doesn't happen with legal products where consumers have legal recourse against sellers that commit fraud. Also, who profits from the black market? Organized criminal cartels. Alcohol prohibition helped turn the American Mafia into being a bunch of run of the mill ethnic criminal gangs into a multibillion dollar criminal empire. Drug prohibition is doing the same for many other criminal cartels, including the Mafia, urban street gangs, and drug cartels in Mexico and South America. Legalization would mean users no longer need to rely on criminals to get what they want.
4. Money. We waste billions in law enforcement expenditures to arrest, prosecute, and incarcerate drug offenders. Imagine if those dollars could be returned to the tax payer. Or at least used to fund other more pressing areas of law enforcement. Plus, by forcing drugs onto the black market, that means the government doesn't collect one red cent from an industry that makes billions of dollars each year.
5. No harm principle. My decision to smoke a joint harms no one other than perhaps myself. Since my decision to use or not use does not infringe on the rights of anyone else, I should be free to make that decision on my own. Some will argue that drug use can lead to acts that are harmful to others, but the fact is drug use in and of itself harms no one. For those harmful acts it can lead to, we have existing laws that can deal with those issues. Driving while intoxicated is often cited as an example, but even if we legalize pot, we still have laws against driving while intoxicated. Legalization wouldn't suddenly make it OK to get high as a kite and then go out for a cruise.