If you meet an overweight person, do you begin conversation by telling them to eat less? If not, why don't you? They obviously get too much calories and may benefit from your advice. And they can always tell you to stop giving them advice if they don't like it.

Seriously, some people have no common sense and/or are just arrogant. Jumping into a situation where you have minimal information and assuming to know best is the hallmark of a fool.