Because the drug is also a sedative, that leads to people being somewhat slower and perhaps unmotivated. What's really unfortunate is that people think the drug is gone when the buzz is, but nothing could be further form the truth. On the other hand, we've never documented a case of a lethal overdose from marijuana, so from the standpoint of immediate toxicity, it's not so bad. I liken it to taking Valium which sedates, reduces anxiety, and impairs learning. There s much debate about that.
Many people use marijuana and can walk away from it.
A Few people literally fall in love with the drug, probably because it reduces anxiety, and there in a situation with a lot of anxiety, and they tend to become dependent. So whether it fits the classic definition of addiction or not, a few become so dependent that it does affect their life. I work with an addiction center and we see adolescents quite dependent. Typically a kid will start using the drug and because of it's anxiety reducing property, etc., and help people go through life and not face their problems, kids slip academically, have more anxiety, smoke more marijuana, and finally end up in deep trouble. However, once they get treatment, they're able to get off the marijuana reasonably easily and in many cases, stay away from the drug completely.