It starts with the 'legal' opiate, which is fundamentally the same drug as heroin. Doctors allow their patients to become addicted to the legal stuff. Then when they get cut off from that they need to feed the addiction with the next best thing. 
Yeah, there are a lot of "weak minded" people out there. And you're one of them, you just haven't had the opportunity to become addicted. A lot of nicotine and alcohol addicts out there. My personal addiction is caffeine.