When you've made a personal commitment to healthier eating/living habits, how people are frequently looking over your shoulder to make sure you're sticking to it, and rarely missing an opportunity to comment if you're not measuring up to their standards ("Why are you eating pizza? I thought you were on a diet.")
Made all the more irritating by the fact that, if you weigh 600 lbs and make no effort to live healthier, you can eat a plate of reheated mozzarella sticks for breakfast and nobody will say a word to you about it. So, really, the comments are never about people looking out of for you - they're about the insecurities of the person who can't help but comment.