The first and most basic reason that marijuana should be legal is that there is no good reason for it not to be legal. Some people ask 'why should marijuana be legalized?" but we should ask "Why should marijuana be illegal?" From a philosophical point of view, individuals deserve the right to make choices for themselves. The government only has a right to limit those choices if the individual's actions endanger someone else. This does not apply to marijuana, since the individual who chooses to use marijuana does so according to his or her own free will. The government also may have a right to limit individual actions if the actions pose a significant threat to the individual. But this argument does not logically apply to marijuana because marijuana is far less dangerous than some drugs which are legal.
The second important reason that marijuana should be legal is that it would save our government lots of money. In the United States, all levels of government (federal, state, and local authorities) participate in the "War on Drugs." We currently spend billions of dollars every year to chase peaceful people who happen to like to get high. These people get locked up in prison and the taxpayers have to foot the bill. We have to pay for food, housing, health care, attorney fees, court costs, and other expenses to lock these people up. This is extremely expensive! We could save billions of dollars every year as a nation if we stop wasting money locking people up for having marijuana. In addition, if marijuana were legal, the government would be able to collect taxes on it, and would have a lot more money to pay for effective drug education programs and other important causes.
The third major reason that marijuana should be legal is because prohibition does not help the country in any way, and causes a lot of problems. There is no good evidence that prohibition decreases drug use, and there are several theories that suggest prohibition might actually increase drug use (i.e. the "forbidden fruit" effect, and easier accessibility for youth). One unintended effect of marijuana prohibition is that marijuana is very popular in American high schools. Why? Because it is available. You don't have to be 21 to buy marijuana -- marijuana dealers usually don't care how old you are as long as you have money. It is actually easier for many high school students to obtain marijuana than it is for them to obtain alcohol, because alcohol is legal and therefore regulated to keep it away from kids. If our goal is to reduce drug consumption, then we should focus on open and honest programs to educate youth, regulation to keep drugs away from kids, and treatment programs for people with drug problems. But the current prohibition scheme does not allow such reasonable approaches to marijuana; instead we are stuck with 'DARE' police officers spreading lies about drugs in schools, and policies that result in jail time rather than treatment for people with drug problems. We tried prohibition with alcohol, and that failed miserably. We should be able to learn our lesson and stop repeating the same mistake.