Not everyone can handle smoking weed. I've seen weed ruin many of my close friends. If you have an addictive personality theres a large chance that you will get psychologically addicted to the bud. Just think of what could happen...
I personally think that weed needs to be regulated by the government and should not be given to everyone. Imagine a world where all the presidents are high. Countries would probably also get into fights over weed and ridiculous stuff like that.