What makes someone die from carbon monoxide poisoning?

Lack of oxygen. Carbon monoxide has a tighter bond to the bloods hemoglobin molecule than oxygen to the point it displaces it. If a person is in an setting where there is some oxygen but a high concentration of carbon monoxide, the oxygen will be displaced and the body doesn't get the oxygen. The effect is the same as having no oxygen available. They suffocate.
Not likely! If you car is parked in a well ventilated area (not in a garage) and your exhaust system is working properly, then you are not going to be exposed to excessive levels of carbon monoxide and therefore you are not in danger of cmo poisoning.