Hey, I just do it for the cool factor, and because my friends do it, honestly
cancer
"Acute myeloid leukemia
Bladder cancer, Cancer of the cervix, Cancer of the esophagus, Kidney cancer, Cancer of the larynx (voice box), Lung cancer, Cancer of the oral cavity (mouth), Cancer of the pharynx (throat), Stomach cancer, Cancer of the uterus...
Cigarette smoke contains 4,000 chemicals: hydrogen cyanide, formaldehyde, arsenic, ammonia and urea among them."
EDIT: There is a delay effect in-between smoking that increases your normal stress-levels until you smoke again, which is one of the marketing tactics.
Just saying.