Imagine this: Your 16 yr old just got his license, for all practical purposes, you no longer have control over what he does in the car. As a protective parent, of course you want him to wear his seatbelt, but as a rebellious teenager, he doesn't want to. Just let him get pulled over a couple of times for it and see if he doesn't start to wear it.
The argument that the government shouldn't tell you what to do is lame because the government tells you not to commit murder, not to steal, not to beat your children, to pay taxes, and lots of other things that you may or may not want to do. To say the government shouldn't tell you what to do is to advocate anarchy. Which most people don't really advocate because they want the government to control everyone else.
To say that not wearing your seatbelt doesn't hurt anyone but you is also lame. Ask your family about that when you get killed. But besides the emotional aspect of that, what about the expense involved if you don't die but you are critically injured instead? Besides the possibility of bankrupting yourself and your family, the insurance company has to pay so everyone's rates go up. If you don't have insurance or money, the state usually ends up paying the medical bills, so taxes go up.
Even if you are a wonderful driver, the other guy out there isn't. Tell me, do you play Russian roulette, too?