addiction is mostly mental, dependence is physical. The part that makes it addicting has to do with stimulation of neurotransmitters, hence why coffee is considered addicting but no one becomes "dependent" upon it mostly because the side effects/physical effects are minimal. this is somewhat of an oversimplification as when one is addicted to a substance one is considered "mentally dependent", but modern psychology does differentiate between mental dependence and physical dependence. Mental dependence would be the feeling of needing a substance to function normally, if you don't get it then maybe you flip out and can't control your anger, that would be mental dependence. Physical dependence is when you don't get dope and you start shitting yourself or if you dont get alcohol/benzos and start seizing.
one of the reasons opiates fuck your body so hard is because you have opiod receptors all over the place, including your stomach and even your ears.
smoking/using cannabis is associated with stimulation in the reward centers of the brain, hence the association with addiciton. addiction can be overcome, but no one is stronger than addiciton.