Theres people making good points in this thread, but also ALOT of 'wrong', and ignorance. Dependence to certain substances, in my opinion, can be inevitable. It's biochemistry, when you put X in your body, a new chemical/biological state Y will be produced etc. Eventually, (and the thing is, 'eventually', will be a different period of time depending on your body) but eventually the new state will become closer to the norm, which can be very, very shitty.
An example is alcohol. It increases a a molecule called GABA. GABA inhibits your nervous system. Slows everything down. Drink enough and elevated levels of this molecule will become the closer to the norm for you. Now when you're not drunk your GABA levels are much lower compared to the state your used to. Since it's affect is to slow/modulate your nervous system, an absence of it can do very fucking scary things. Basically your nervous system goes crazy in it's uninhibited/low GABA state, neurons firing uncontrollably etc.
The result, depending on the severity of the condition, can be seizures, hallucinations, coma, death... Read about peoples stories of "Delirium Tremens", something that happens to around 15% of alcoholics who try and quit. The stories are sometimes fucking terrifying. People seeing spiders crawling over them, shaking uncontrollably, seizing, feeling like their heart may just stop....
Quitting an addiction has more to it than fucking willpower. In this case willpower alone, without trained medical staff and equipment, may just get you killed.