Of course, not everyone believes that marijuana should be illegal.  However, if we assume that it should be, there are a number of reasons that can be given.

First, we can argue that marijuana is immoral.  In this view, it is not right for us to use chemicals to give ourselves pleasure.  It is not moral for us to abandon our rationality and our control and make ourselves intoxicated.

Second, we can argue that marijuana is addictive.  It is not highly addictive like drugs such as heroin, but it can cause users to become addicted.  Nothing that is physically addictive is good for us.  Therefore, marijuana use should not be encouraged.

Third, marijuana has other negative health effects.  It has negative impacts on our ability to pay attention, to remember, and to learn, even days or weeks after we are no longer “high.”  Again, a drug that does this should not be condoned.

Finally, marijuana use has the ability to lead users to “harder” drugs.  People might get started smoking marijuana but they might then move on to things like cocaine and heroin that can really destroy their lives much more easily than marijuana can on its own.

For these reasons, one could argue, marijuana should continue to be illegal.