I didn't read all the previous posts, so sorry if this is repeated.
I think most people start for one of two reasons: (1) Spite. My dad started for that reason. His friends were smoking and his mother caught them. She yelled at my dad, even though he didn't smoke anything. So out of spite he started. (2) To fit in. Why else would people pay $60 for a pair of name brand jeans when better looking ones are $25. Why else would people pay $150 for basketball shoes when some $40 shoes are better for basketball. Why else would do teens do so many of the things they do. They just have to fit in with their friends and thus they start smoking. Sure there are probably other reasons, but that covers the majority.
Most smokers I know say they continue smoking since everyone needs something to calm themselves down in stressful times. "Hey everybody needs a vice" is what I hear. Then I look at smokers and non-smokers. In stressful situations the non-smoker only has to deal with that stress. The smokers have to deal with that stress AND they have to deal with intense cravings for a smoke. Thus the smokers have twice as much to deal with. They smoke a cigarrette and it definately helps - their stress level is cut in half - now they only have the original stress to deal with. However if they never smoked, they would have felt that way to begin with. I've never heard a non-smoker say "everyone needs a vice".