Car insurance laws exist for liability reasons. You get in an accident, it's your fault, your liability insurance pays for the damage you inflicted. I've never heard of any laws mandating coverage for your own vehicle.
Health care is different. You can only damage yourself by not having health insurance.
You can also do damage to the system as a whole, because most hospitals are required to treat emergency patients knowing they will never pay. Where do you think this lost revenue is made up? Jacked up rates for everyone who DOES pay. The health care system does need a big overhaul, everyone should have access to healthcare, but Obamacare is complete garbage. How do we tell people who are already struggling to get by that they are now required by law, and face huge fines if they don't pay into our corrupt health insurance system? We don't need insurance companies... at least not on the scale that they exist in 2012. We need state-run, single payer programs. If you have the means to purchase more premium care through insurance that can always be on the table as well.