What I was told at my daughters rehab is that everyone is born with the same amount of endorphin receptors, but not everyone has the same amount of endorphins (natural opiates). It's why people have a higher/lower tolerance for pain. When people with low endorphins take opiates for pain, their brains finally feel normal.
When anyone takes more opiates than necessary, or for prolonged periods of time, the brain develops a dependence because it's produced extra endorphin receptors to handle the opiates. They need more to manage pain or be pain free. Those extra receptors will go to sleep after a period of abstinence but they will always be easily wakened.
Having low endorphin levels also leads to depression besides dependence or addiction. Genetics play a huge part.
It took me a while to understand and accept the difference between addiction and dependence. They are both a medical disease because the brain isn't functioning correctly. Addiction is also a spiritual disease because the conscience/consequence isn't functioning correctly.
When I looked over my children's family tree, I saw scattered depression and alcoholism on my side and my husband's. Diabetes is dominant on my side, and that ties in neatly with alcoholism. We can't handle excess sugar in whatever form.
Alcohol, caffeine, nicotine, sugar, and all mood altering drugs share the same pathway in the brain.
I'm positive my son has a sugar/carb addiction and is headed towards diabetes if he doesn't manage it soon. My daughter has an opiate addiction. I'm addicted to nicotine. My husband doesn't overdo anything because he's very aware of his genetics. It's in all our genes.