The obvious reason is Nicotine. It's effects are described
But there are other, reasons for adults indulge in the vice of smoking cigarettes, and these reasons characterize two different groups:
First, the individuals who smoke simply because they enjoy life. This person enjoys fine food, good music and in this case, the sensation of warm, coarse smoke slowly entering and exiting his lungs. He enjoys manipulating the fumes, he enjoys the fondling of the cigarette, the stuffing of the pipe and the simple, yet entrancing, flicker of a match.
But why choose cigarettes when more fulfilling alternatives like cigars, pipe tobacco or even joints exist? Cost, accessibility and practicality.
To characterize the second group we must investigate why they began smoking in the first place. Bullet pointed:
1. Curiosity.
2. Group pressure.
3. Pop-culture and other phenomenons inspire enough to begin smoking. I.e. "coolness-factor".
Most of these are totally excusable reasons to begin smoking. It is part of human nature to be curious, to bow under to group pressure and to be influenced by the culture around us. Where the chain breaks is when the teen-smokers grow up and then continue smoking. They do not smoke because they enjoy it, they smoke because they have not had the will/strength/incentive to quit.
So, the question is not whether smoking is good or bad and should we allow/disallow it, the question is rather, how do we create a set of rules that fits these vastly different groups. There is no reason to punish the first group as they are more than aware of what they are doing and could, but will not, quit anytime. Why take away their freedom to indulge in something they enjoy? The second group, on the other hand, are not able to take care of themselves and need consumer-protection. They endanger themselves and their surroundings (Think of the children!), not because they enjoy what they are doing, but simply because they are addicted.
So, what do we do? If we disallow cigarettes, we punish the first group. If we keep allowing cigarettes, we let the second group punish themselves.
As I see it, the solution is to ban nicotine. From cigarettes, from pipe tobacco and from anything else except medications. Those who enjoy life and smoking will not be affected since they do not smoke because of addiction, but rather because of an appetite for life. The second group, those who smoke because of habit and addiction, will find it substantially easier to quit.
The problem with the nicotine-removal [over-time] idea is that tobacco users would continue t seek nicotine up to the level that provides a satisfactory dose. This is the reason why ‘light’ cigarettes are such a fraud. With light cigarettes, the smoke is diluted with air drawn through ventilation holes in the filter, but smokers respond by taking in more of the weaker smoke to attain the nicotine they need. The machines used for measuring cigarettes do not respond in this way, so the light cigarettes give low tar and nicotine readings on machines but unchanged doses to the smoker. Switching from full-flavour cigarettes to light cigarettes is a little like trying to reduce alcohol intake by switching from wine to beer.
With light cigarettes the tobacco is almost the same as in conventional cigarettes, and an attempt to reduce its nicotine content would make matters worse: the smoke would not be diluted, but it would have a lower concentration of nicotine. This means smokers would be taking in more undiluted smoke to attain the nicotine they need. If this happened their toxic exposure would increase and the health impact would be serious — possibly adding millions to the expected tobacco-related death toll.
So, to avoid a period of half-assedness, nicotine should be banned immediately and entirely (medically supervised to ease users transition). And no, people would not flood to the illegal channels as they do during prohibitions, as nicotine does not actually do anything substantial but make you addicted.
post scriptum: It should be noted that the two groups described above are also recognized within drug users and consumers of alcohol. Sadly, the same solution to the problem is not available.