Helmets saved me and my spouses life back in 2000, drunk driver. Even though I lived in CA at the time where they're mandatory I still wear em in CO where they're not.
A good friend of mine lives in IL, a helmet free state. If he had not been wearing his helmet in Oct of 2016, he would either be dead or severely brain/spinal injured. 45 mph on country road, hit gravel patch (he thinks), sideswiped oncoming car, ejected, landed on road, and limped away. Helmet had a nasty gash on it. To each their own. I would never force my opinion on someone else, after they say "no" the first time.