Actually I asked the same question to some of my friends. They are aware about the danger of not wearing a helmet, and the excuses are: Helments are hot and uncomfortable, it gaves headaches, mess up the hairstyle and so on... Please don't think about the excuses and wear a helmet every time you ride.
I wear a helmet because California state law says I have to. If I go to another state that has no helmet law, I take it off. I'm not so convinced that it would save my life, I know plenty who have died with a helmet on their head, and it does interfere with vision and hearing. Also it detracts from the enjoyment of riding.
If a motorcyclist doesn't care about their own life, I wish they'd take into consideration their family members and the drivers of other vehicles on the road. If I ever accidentally mow down a motorcycle with my car, I would certainly like the motorcyclist to survive, and I know their family members would like them to, also. How terrible it would be for a driver to have to live with killing a person, and how terrible for their friends and family to lose a loved one.