Well, a few random thoughts (since it is nearing spring as someone mentioned);
1) Grew up wearing one dirt bike riding. It just feels right. Do you know of any dirt riders who DON"T wear helmets? I think they know what falling without one does to a head.
2) When I see a (I know...my stereotypes)
* clean shaven rider w/o helmet I think Newbie trying to be cool.
* an older, bearded rider I think ok, experienced rider who just doesn't want to wear a helmet. His choice.
* a helmeted rider on a cruiser (like me), I think doesn't look great but at least he is doing the smart thing.
3) Experience - Bicyclists wear them all the time racing...I slammed my head into the ground at 23MPH 5 years ago and my bike helmet was split X&Y. Saved me I believe.
4) I respect a person's choice NOT to wear a helmet.
5) I like my hair matted rather then windblown.