Generally members of the public are fairly polite. They don't pass comment on personal things and tend to err on the side of pleasant when they do comment. Also, they don't usually speak to total strangers just because they can.
Unless you're pregnant or have just had a baby on which case it is deemed entirely appropriate for strangers to comment on your age/size/bump/number of children and subsequently comment on your baby as to whether they think they are big/small/the right or wrong gender. My daughter was a month early and so it was obligatory to comment on her smallness. More than one person asked me on all seriousness if she was in fact a doll.
Medical conditions are apparently a free for all generally. Nothing seems to be quite as hilarious as an eye patch. Tee her, eye injuries are such a laugh. Pirate jokes are always welcome. How about a stookie, or crutches, or other visible malady? Clearly anyone would want to discuss their medical history with total strangers.
We have a dog; he is a greyhound, a large greyhound at that, and yes, he's a big dog. He comes on the school run occasionally and almost every child who claps eyes on him comments on his size. Which is fine. What is weird is adults saying to me in a slightly disapproving manner that he's so big. Or they say to him "aren't you a big boy?" to which he responds with a hopeful sniff for a treat and not much else, being, as he is, a dog.
I'm not sure how people in the public eye cope with open questions being asked by everyone of them. It's appalling the way people cast assumptions about people and why they are single, or childless, or why they dared to go to the post office without wearing full makeup.
Having said all that, I do worry about that which I don't say, but which may be written all over my face due to what I am thinking. You don't actually know exactly what your face is giving away. What shows on your face when someone is wearing way too much perfume and you're breathing it in? When you see someone and are racking your brain to remember who you know that wears glasses just like them? Or your desperately trying to remember their name or even how you know them? When someone says or does something pretty innocuous but that happens to be a pet peeve (like bad grammar, for example)? When it is really important you don't offend someone but they say something actually awful?
I think this may be another ex smoker thing. When you are smoking your face has something to do. When left to its own devices, your face can get you into a whole heap of trouble.
Of course for some people, they just blurt out what occurs to them. I suppose they don't have to worry about such trivialities as being misunderstood or inadvertently doing anything.