Me 3! Yes, my moms and alcoholic/drug abuser. All 3 of us ( My 2 brothers and I ) ended up with opiate / other problems. We all had a natural tolorance as well. My oldest died from it. My brother is doing bad. I detoxed a little while ago. i feel good but not too excited both of my brothers detoxed and did the rehab thing numerous times.
To answer your question, sometimes people are born with natural tolorance. A Girlfriend i once had , she had it even worse than i did. I started out with oxy, she took 10,nothing, 20, nothing, 30 and she finally got high. Weird for someone with NO tolorance. So some of us are just born fucked with natural tolorance and prone to addiction.