Opiate receptors are not the sole mediators of analgesia. But, as you intimated, I think it’s fair to say that the answer to your question boils down, in essence, to differences in cell surface receptor function or differences in downstream intracellular pathway components that are activated by such receptors.
My 'wild' years built me up a tolerance to intoxicants, both physically which lessens over time, but more importantly psychologically which doesn't. It always take more stuff to work on me. I've had nurses in my room after surgery telling my family "he can't be awake and talking now like he is, it's just not possible, and I'm scared to give him more" a Doc would walk in and say 'hit him again".
OH I don't know, I used to smoke and had a pretty high tolerance for nicotine but having quit for years I now have a very low tolerance. Your body will downregulate (OR should) the pathways for metabolizing drugs if you don't continue to use the drug.
The question is more - why do some people have crap receptors - and this can depend on a multitude of things
