Why do complete strangers feel it's okay to comment on someone else's habit?

I’m a smoker, and a conscientious one: if I’m smoking in public, I do it in designated areas and only if there’s plenty of air and room around me that nonsmokers can avoid it. Now, the air in Los Angeles is teeming with pollution, but I don’t go up to drivers and tell them to cut their engines. Trans fats clog arteries but I don’t approach overweight people and ask them why they’re snacking on a Twinkie. Yet for some reason, complete strangers seem to like to come up to me and tell me how bad smoking is for my health. Today, some guy walked out of his way to tell me that I was killing myself (In reply, I asked him if he wanted a medal for stating the obvious, but that’s besides the point). This is far from being the first time this has happened to me.

Similarly, a few weeks ago I was with a friend of mine when someone she didn’t know approached us and said to her “It’s called ProActiv.”

So my question is: Why do people feel it’s okay to approach complete strangers and comment on their habit/physical characteristics/whatever? Do they really think they’re going to make a difference in someone’s life, or are they just out on some type of moral crusade? Neither?