The problem with cannabis is not so much people using it and causing harm, but the trafficking (selling) part of it.  Dealers fly it in from Canada and Mexico and it gets sold all up and down the U.S. which creates a lot of crime.  Now that we're in a recession, it's expected that crime will go up because people are turning to other ways of making money. 
I think marijuana is a lot less harmful than a lot of other drugs, but it's still sold the same way as cocaine and heroin.  And a lot of people use marijuana as a drug, instead of every once in a while, I think there's more hard core marijuana addicts than ever before. 

That's true because then one person always ends up regretting it.  If you tell them early, assuming they're a good person, they'll probably admire your decision and hopefully support you. 

This is part of way the decision to stop drinking is a hard one to make.  But it's sort of a good thing because you realize who your true friends are.  I mean you don't want people using you just to pitch in for alcohol, or they only like being around you when you're drunk.  That's not cool.