The hidden assumption you make is discussed as the Preference Satisfaction Thesis in the philosophy of welfare economics, it's the idea that people (subjectively) prefer what is best for them. Almost everybody agrees that the thesis is generally speaking false, although some people would argue that you should accept the preference satisfaction thesis as a rule of thumb if you are an ethical anti-paternalists.

Anyway, to cut a long story short, you could be like the smoker who prefers to have another cigarette even though it's bad for him. Not only that, it's even possible to find examples of people being wrong about their own subjective experiences. For example, people sometimes have cyclic personal preferences and one of the arguments against them is that their subjective feelings are wrong.