Let's hypothetically pretend you're a father (I know, difficult, no woman would want to have a child with a libertarian). You're driving your truck with a gadsten flag on your way to defend your gay weed farm from cops or something, and get into a collision. Because you aren't wearing a seatbelt, your morbidly obese body flies out of your windshield and you die upon hitting the pavement and splattering like a tomato.
Any children you hypothetically have now must grow up without a (fat and retarded) father. Everyone can agree that people who grow up missing a parent (or both) usually end up fucked up. So, in interest of having a society comprised of healthy, functioning adults, seatbelts are mandated by law to decrease driving fatalities.
It is in the common interest to have safety regulations. Health insurance is lower because traffic accidents are safer. Fewer skills/trades are lost due to premature death. Family units aren't broken, producing productive members of society. The act of not dying is another notable benefit. More time for you to hit your bong and annoy smart people with.
I'll reiterate. The government has placed safety mandates like seatbelts in order to prevent premature death in the event of a traffic collision. This has benefits like you not losing the only plumber in your inbred town because he died in a crash early, or your habsburg children growing up with both parents.
Not wearing a seatbelt is childish, we all know you're doing it just to protest MUH GUBMINT in the most weak and discreet way possible. They can hardly be called an inconvenience. If a cop pulled up and told you to put it on, you'd obey him and put it on like a little bitch. You libertarians are all pussies, all talk no action. I'd punch your head off.
By all means, I encourage you to continue not wearing it, preferably driving like a retard because driver's licenses are for bootlickers and blah blah its my car I do what I want blah blah it's my property. Hit a tree.