Heroin is an opiate, and it causes a severe physiological and psychological addiction whether the user is dependent on it for self medication due to other disorders or not. To cease a heroin addiction generally requires clinical intervention with a structured recovery program. Benzodiazepines? Ceasing heavy abuse can cause death because they are so physiologically addicting.
THC? Studies and anecdotal evidence all confirm it's minimally addictive. As I said, quitting a caffeine habit would be comparable on the extreme end. Anecdotally, I know people who use THC recreationally and they have no problem stopping use when they need or want to. Withdrawal is minimal, people notice it's a little harder to get to sleep, especially if they use THC as a sleep-aid, and maybe a few days of low mood as they adjust to processing raw emotion again. If someone cannot physically quit THC, it's an extremely rare/unique situation, or there's something else about their situation that you do not know.