Why is it hard to convince some people to do what doctors tell them?

Denial, inconvenienc: Most medical advice will suggest the patient change lifestyle, eating habits, exercise habits, spend money, suffer inconvenience. Most involve delayed gratification - if you do this you will feel better in the future. It is easiest when the suffering is obvious - e.g. Not too hard to get an arthritis patient to take anti-pain medications. Compliance is a major issue for most.