Many many people have a very difficult time managing ctheir weight. I think doctors need to take this issue seriously, but compassionately....otherwise overweight people will simply stop going to the doctor all together. No one wants to be harassed/humiliated over something they already know to be true. They know they're fat. There is a difference between telling someone you're obese and you're gonna die and saying something like, I see you're having trouble managing your weight. Let's talk about it and see what we can come up with to help you. Being supportive and encouraging gets alot more done than being a jerk.
Having waged a nearly life-long battle with being obese (currently winning)I'm aware of exactly how fat people feel. I still have no problem with a doctor saying "if you don't lose this weight you're very likely to have heart problems, diabetes, etc. and may die young." I don't think a doctor needs to sugar coat it, because the majority of obese (heck even just overweight people) want to ignore the fact that it's a problem or is likely to cause them problems.