Why would someone try to hide that they started smoking again?

Why would someone try to hide that they started smoking again?

I've known someone for about half a year. When we met he decided to stop smoking and for a few months was even successful. (Just to make IT clear we are not in a relationship and quitting was his choice I had nothing to do with IT). I was happy for him. However, recently I started suspecting he went back to IT as he stopped mentioning how long he could manage without smoking, he started going out in the cold and hiding behind the corner of the building in order not to be seen or spending long minutes inside his car before finally coming out. I know it's not my place to criticise him and I wouldn't do IT, but I feel confused as to why didn't he say he started smoking again?

Why would someone try to hide that they started smoking again?
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