You are noticing a fairly common response in some patients. Their beliefs and expectations dictate their pain more than the actual condition. Many studies report similar findings. It doesn't seem to matter what the problem is: irritable bowel, angina, fibromyalgia, headaches, and so on. How the patient interprets the pain is the key.
Researchers also report greater pain and drug use when symptoms occur after an injury or trauma. You may want to talk with your father and his doctor about his need for drugs and the best kind to take for his symptoms.