People wouldn't care except that your "personal" decision affects them.
(1) Everyone pays for the public health care system e.g. hospitals. So if you're increased risk of serious disease results in increased visits or treatments then that's my problem as well.
(2) Everyone pays for insurance. So if you're making premiums more expensive then that affects me too.
Yes it's perfectly logical under a single payer, universal healthcare system. So the reaction by some is: this is exactly why we should not have universal healthcare.
> Everyone pays for insurance. So if you're making premiums more expensive then that affects me too.
It muddies the water quite a bit by having a marketplace of different private insurance companies. If you see me doing something risky you have no clue whether or not I'm even covered under the same insurance company as you. So maybe I am, maybe I'm not affecting your premiums. Single payer universal healthcare removes all doubt.