It seems logical to me, you never know what is the spark to cause someone to change a habit. It could be a little kid telling his mother "Look at that fat guy" and you overhear it. It could be a near-miss while driving drunk. It could be Johnny Carson dying of emphysema.
I quit drinking in October 1997 - it took a near fatal bout of pancreatitis to provide the spark. I quit smoking in September 2002, after landing in the hospital with chest pain (notice a pattern here?).
Currently I'm fighting my last vice - getting my weight and diet under control. I had been doing Atkins for a while and walking, and I had lost about 70 lbs. or so (after gaining at least 50 when I stopped smoking), but had slacked off badly and was again eating all the wrong stuff. The spark that has gotten me serious about dropping the last 100 lbs. or so I need to drop - a diagnosis of Type II diabetes and sky-high cholesterol and triglyceride levels.
Some people can smoke 2-3 packs per day, eat four fried eggs and a half pound of bacon every day, have their most strenuous activity of the day be reaching for the remote, and still live to 90. I'm not one of them.