For some people, it seems they feel the need to pick at the minor details in life and if they get those things right it'll add up to a greater righteousness, sort of a gestalt religion. Once those people feel they have those minor things perfected in themselves, regardless of their own failings, they see fit to correct those same things in other people. I guess it gives them a sense of control over their surroundings and boosts themselves up to be able to manage others.
Whatever the reasoning is with it, if it's addressed to me personally I usually just shrug and tell them I have lots of imperfections so it comes as no surprise that they've found one. If it's directed at someone or something beyond my control... well, that's beyond my control. In other words, mind your own beeswax, eh?