Usually when you visit the doctor, the hospital personnel and medical staff all seem to be on the healthy side. There may be a few exceptions but hospitals seem to be full of doctors and nurses and other who have narrow waists, low BMI and you never catch a whiff of cigarette smoke near them. I can say that all my doctors are a healthy weight - except the pregnant ones.
So when they start talking about weight and healthy eating and exercise, I do give them credit that they must follow that life style as well - they must walk the walk as well as talk the talk.
I know about twenty years ago, I was treated by a nurse who had a pack of cigarettes in her pocket. I never see cigarettes around any more. The hospital now has a no smoking campus, so you see dwindling numbers of smokers hiding off the edges of the property enjoying their morning cigarette.
But what if you saw a doctor who was obese, reeked of cigarette smoke, and had the unhealthy pallor of a couch potato who's idea of exercise was another trip to the refrigerator? Shouldn't your doctor practice what they preach? Doesn't it give them a bit more credibility? What if your dermatologist was sporting a sunburn? I think if my doctor was an obese, chain smoker I wouldn't give any of their advice as much credit.