I can follow instructions too but I wouldn't change the brakes on a car without supervision from someone who knows what they're doing. That's a safety-critical repair.
More or less this is the point of my reply -- cars are crossing into a threshold where life or death quite seriously hangs in the balance on some repairs.
It's not just the failure of the system itself I'm worried about though, the _process of making certain repairs_ could kill you in many fields, not just automobile repair.
Working on high-current electrical systems is dangerous, working with sprung weight can be dangerous, working with flammable vapors can be dangerous. Automobiles are one of the few examples, though, where you're _surrounded_ by many of these dangerous sub-systems while attempting a repair on something that's otherwise fairly harmless [brakes, changing a tire, checking fuel pressure, replacing a battery].
The interactions between such subsystems is where the danger lies -- and without the domain knowledge of an auto mechanic, you could easily be left unaware of the dangerous interactions between these subsystems.
