I think that the fact that cannabis is such a benign drug can actually be a problem. What I mean is that, if you compare heavy amphetamine or alcohol or pretty much any other drug, heavy, regular use tends to lead to negative health, social, mental, financial etc consequences. I was a teenager smoking all day every day, but it didn't really have a negative efect on me- I did well at school, had a normal social life, never spent all my cash. If I'd been drinking that much, I would have felt like shit all the time, lost friends due to drunken behaviour, had no cash, done shit at school. The problem is that because of this you can very easily slip into really heavy use. I remember being shocked when one of my friends told me that he smoked as soon as he got home from school; in a couple of years I was waking up with a couple of shotties. I became dependent on it, I just hated not having dweed. Now that's not great in itself, but the real trouble was when weed started making me anxious, paranoid, and just not being much fun. But I was so used to blazing all the time, I didn't really like being sober- I was just bored and miserable all the time. So I started drinking maybe half a bottle of rum a night to try and compensate. Suddenly, I was missing all my lectures, broke, feeling like shit all the time, making a fool of myself and putting on a lot of weight. Alcohol just can't be used like cannabis can without fucking your life up, but when dope doesn't do it any more you just want to get fucked, and you have to learn how to be sober.
I think this post is really long; sorry, I'm higher then the sun and rambling.