Gary Taubes has been talking of this. In his lecture recently he mentions slim people telling overweight people that if only they would eat as I do then they'd have no problem. This is particularly true of slim HCPs.
The conversation seems to go both ways.
On one occasion, I have had a passing acquaintance tell me I am "too thin".
On another occasion, I have had a very overweight HCP tell me I need to watch what I eat because my BMI had gone up (from 20 to 21) in the last year.
I have also had meat eaters tell me I am doing myself harm because I eat veg and fish.
Diet is a very complex issue because we are all different - our bodies react to different things, we have different allergies and intolerances, we have different abilities to exercise, we have different lifestyles, we are motivated by different things, we are emotionally sensitive to different things, ... and all these affect what diet works for us.
Likewise, we are surrounded by different people. Some extend their motivations upon us, some keep quiet, some want to help, some think they are helping and some are ignorant. But ignorance should not be confused with a lack of caring or stupidity.
Personally, I will politely decline food I don't want but I don't feel the need to explain why. Most people get the message eventually and stop offering jelly sweets or ham or ... If they want to understand my food choices, they will ask ... and some do ... but I don't feel I need to explain every time I decline some food.