Why Am I Having Severe Oxycontin Addiction?

I am a 32 year old male and have been opiate dependent, abusing mainly pharmeuceutical opiates, starting at the age of 18. I am currently taking Subutex. I have slowly tapered my dose, where I started at 8mg x 3 tabs (24mg) daily, down to, between 8mg-12mg per day. I began using opiates, after severily breaking my arm, where I soon found myself graduating from hydrocodone, to a severe Oxycontin addiction. Between, the age of 19 to 22, I was being prescribed anywhere from 4 to 5 80mg Oxycontin, but found myself using more than prescribed, which I further abused by snorting the majority of them. I soon found myself without a doctor and started "methadone maintenance" at the clinic, where I was given my maximum dose, at 275mg daily. Within the next two years I found myself 100 pounds heavier, an enlarged patuitery gland