Remember kids... always wear a helmet. (The almighty bikeradar helmet thread)

Many people cite different reasons, for some people undoubtedly fashion is the reason, other reasons may be :

comfort,
convenience,
studies showing worse behavior of driver towards cyclist wearing helmets,
believe that while a helmet may offer some protection in some accidents the chances are very small, i.e the same reason people don't wear a helmet walking or running.
freedom

Someone really should invent a hardshell balaclava. the ultimate winter helmet