When one is under tension or is feeling confused or nervous or not having anything to do, one tries to smoke and get some relief or happiness.
SMoke particles collect inside lungs, get deposited along the paths they travel. The mouth smells bad. Due to heat, the lips become dark or black. Due to pollution of the breathing paths, lungs do not function well. It is possible that one may get lung cancer, mouth cancer.
It is not only to the smoker, the other people nearby, who are passive smokers, also get affected.
Further, the useful oxygen in the nature gets reduced. The heat from burning tobacco contributes to global warming even if it is very small.