Actually - it is addictive, but it isn't the actual weed that is - its the habit, who you're doing it with or not with - when you do it... thats the addiction. I'm a purely social smoker - I only smoke with others. and that means that if we're not smoking (with someone I'm used to smoking with) I get bored and start craving it. Yes, I can ignore it - as I don't have an addictive personality, but I have a roommate that does and he can't go more then a couple hours without smoking because it helps keep him calm.
and I've seen how some of my friends who would smoke regularly would act slightly different when they aren't smoking and then smoke - then see them act completely different personality wise when they haven't smoked in months. Example is actually my sister - when she was smoking all the time and then on the days she wouldn't she'd be easily irriated and easily annoyed vs when she would go months without smoking - she'd have a lot more patience.
it affects everyone differently, and it depends on the person on if its addictive or not. The actual 'drug' may not be - but your habits make it so.