Why do people start smoking if they know it is bad for them?

If people knew a definitive answer for this, then no one would ever smoke.  Smoking rates among youth in America have been in decline for the last twenty years, primarily due to checking if someone is old enough to smoke and the cost of cigarettes.  Smoking rates have increased in Russia and China, however, due to a lack of governmental regulation.  R.J. Reynolds, a leading tobacco company, makes more from their sales in the developing world than they do in America.  

Young people look for things to make them appear older.  Cigarettes have always had that ability.  Some think that it makes them look cool, and others do it in order to go against authority.  Many families have smoked for generations--the child only sees this as part of what they are expected to do.  Once the young person starts, it often becomes ritualized with stressful behaviors such as studying for tests or driving.