No one can say exactly why a person might respond better to one combination of medicines than another. It depends on brain chemistry, inherited factors, emotional state, and other factors. Many medical conditions require trials with different medicines until the most effective therapy for that particular person is found. The reason doctors shy away from patients tolerating pain meds is concern about addiction. There are addicts who complain of pain and go to different doctors to get a lot of pain meds. It is hard to know who is having real pain and who is trying to get meds for their addiction. Narcotic drugs are also closely regulated and doctors who prescribe a lot of them may get reviewed by their medical board and even lose their license. Effective pain management requires contracts between doctor and patient and close monitoring.