Well, I just want to share answer which I got when I interactedsome chain-smokers and asked them the same question most of them replied: “yaar aadat pad gayi hai ab choot nhi rhi hai”.

People get addicted to it and its one of the biggest reason why they smoke knowing it's harming them inside. There are people who treat cigarette smoking as a stressbuster and smoke to make their body active at the time of performing any activity which requires physical or mental strength. 

However, cigarette smoking or any type of smoking only affects a human body in harmful ways. Till now I didn’t find any intellectual saying anything ardent about smoking.