I think people should wear helmets when cycling, especially children, but sometimes I forget to put mine on.
My girls wouldn't think of going out without theirs, because they've always "had" to put it on. But because they weren't the norm when I was growing up, I have to think twice about it.
Like when they first brought in compulsory seat belts, a lot of people didn't see the point, but now it's second nature to buckle up -well, for all but a few.
I have a friend who is only alive because he had a helmet on, and he makes a persuasive argument (with graphic photographs to illustrate a point!)