..in this story in the Daily Fail,
I like this one:
"Smoking psychology is very complex. Everyone knows it is dangerous yet people smoke. Firstly humans have evolved to take risks and to negotiate risks via cause and effect. If the effect is not immediate to the risk taking (especially if the effect is decades away) the brain cannot process the risk. instead people quit for social self-consciousness reasons, financial reasons or aging increasing the likelihood of imminent effect. if humans repeat a dangerous behaviour to no immediate adverse effect the brain learns that it is risk-worthy and safer with each successful attempt."