We need to remain ‘consistent’.
Each of our individual consciences needs to remain in ‘one piece’.
To preserve its self-esteem.

Hence our tendency to rationalize away our mistakes.
Our past decisions which had been proven to be less than optimal.

Hence our tendency to uphold our already ‘adopted’ beliefs.
To discard any new information which contradicts our past conclusions.

The process of ‘selection and discarding’ followed by a robust ‘defensive’ rationalization is almost instinctive. In no way completely conscious.

No one in their right mind can pretend that someone defending their smoking habit is fully aware of what’s going on inside their heads.
That rationalizing away the higher probability of a smoker to develop a cancer is behaving in a fully reasonable manner.

Unfortunately, rationalizing away bad habits is the smallest manifestation of bias.
A more important, and malignant one, is the tendency to impose upon others our own conclusions.
To force others to give up smoking because we’ve reached the conclusion that smoking is bad for us.
To interpret other people smoking – wherever nobody else is affected by the smoke, as a slap in our faces. As an insult to our intelligence.
How does that guy dare to act contrary to what I believe to be proper behavior?