As a physician, you should know that using opioids to experience euphoria and pleasure is drastically different from using appropriate doses to combat the effects of pain. It's the euphoria and pleasure that triggers operant conditioning and the accumulation of ΔFosB that occurs in the vast majority of addictions[1]. Only after receptor downregulation does withdrawal avoidance become a factor, but once that occurs, it doubles the strength of the operant conditioning.
Use != Misuse, but use for pleasure is a direct precursor of misuse.