While there are certainly other influences that can lead a smoker into the habit, peer pressure is one of the biggest. A large part of the reason peer pressure comes under scrutiny is that one of the groups most likely to begin smoking -- young teenagers -- is also one of the most susceptible to peer pressure.
The awkward years between pure, dependent-on-parents childhood and independent young adulthood are marked for many by frustration and insecurity as status, family roles and physical changes happen in a hormone-addled flood. Quite often, this is the time of life when young people rely most heavily on friends of the same age for social support and affirmation.
Enter cigarettes. If a child in a social circle starts experimenting with tobacco, it's all too easy for him or her to lead peers into smoking as well -- the smoker doesn't want to be alone, and the not-yet-smokers don't want to be seen as afraid to try something risky or boundary-pushing.