Prolific weight loss coach Adam Gilbert explains it here:

For example, a smoker could rationalize their behavior by believing that few people get sick from smoking, it only happens to people who smoke more than they do, if smoking doesn't kill them something else will, they'll quit next year, it's too expensive to quit, or they only live once and they deserve to smoke. In essence, they either need to take action (quit smoking and get whatever help they need) or they need to make themselves feel better by rationalizing their smoking—which is what most people do).