Some people can't handle it, but does that mean it should be prohibited for everyone? If it wasn't the ganja, maybe something else would have screwed up your nephew's life. Bob Dylan and Paul McCartney don't seem to be any worse off for their lifetimes of drug use. If you really want to mess your mind, become an alcoholic. And it's legal, too. Or a boxer. Or go fight Uncle Sam's wars in Vietnam, Iraq, or Afghanistan. And come back with PTSD. You didn't really need those legs, did you? Or live the rest of your lives on the street because the government doesn't need you anymore.
And let's make laws so that the Sodomists can get married. And let's dig ourselves $14 - $16 - $20 trillion dollars in debt. Then everything will be all right.