The government has no right to enforce marijuana laws.
There are always reasons why laws exist. While some advocates for the status quo claim that marijuana laws prevent people from harming themselves, the most common rationale is that they prevent people from harming themselves and from causing harm to the larger culture. But laws against self-harm always stand on shaky ground—predicated, as they are, on the idea that the government knows what's good for you better than you do—and no good ever comes from making governments the guardians of culture.