If I'm not harming anyone else by not wearing a seatbelt, then why should I not be allowed to decide if I want to wear one or not?

When you are involved in a car crash while not wearing your seatbelt, your injuries will likely be worse than if you had been wearing one. However, you are not the only one affected. Almost everybody in the country is. Hospital costs go up and everyone on your insurance plan (and other plans) have to pay for your injuries.

I am not trying to preach to you. I sometimes drive around without a belt on, but I'd say 80% I am wearing one.