to piss someone off.
Yes, that was my reason for starting, because my ex best friend was so anti-smoking... so when we fell out, i started smoking just to piss her off.
i was 17 and it was stupid, but then you think you're only doing it socially, then something happens, and you get angry or upset and you think I WANT A FAG, and you do, and then you're not just smoking socially, you're smoking on the walk to college, when you're out with your friends, and then you're hooked.... and your BF is throwing a packet at you as soon as you get in his car because you're snappy and it chills you out.
For all its unhealthy, its still a really good stress reliever, and the only time i think about it now, having given up 3 years ago, is when i'm stressed and all i want to do it stand in my garden and smoke, because its somehow calming, anxiety relieving...