Anyways, a better question is why people START smoking cigarettes. Most people who currently smoke do because they became addicted and it's a habit/routine now. There are many different reasons as to why one would begin to smoke, but continuing to do so is the result of becoming addicted. I know some exceptions to the rule, friends who only smoke when they're drunk or whatever, but for the most part it's due to addiction.
I started cause I was just entering my rebellious stage at age 11. I thought it went hand in hand with starting to drink and smoke weed, so I smoked cigarettes as well. When I was fifteen and decided maybe I should quit for sports (stamina was getting shitty) I realized how hard it was and that I had become addicted.
That's why I still smoke.