weed can totally be addictive and certain people (addicts) clearly can get addicted to it. there's 2 components to any addiction: 1) the physical addiction and 2) the mental obsession.
With weed, the physical addiction is not nearly as strong as something like alcohol or opiates. But the mental obsession can be just as strong. And there are definitely people who are addicted to pot because of the mental obsession.