i dare say people start smoking because their peers do it.
i started because both my bandmates are smokers, and it got pretty boring to sit in the studio alone while the other two went out for smoke breaks.
i used to smoke only then, then i started working at shangri-la hotel where smoke breaks were a great time to bond with your superiors and co-workers without the pressures of work. made lots of friends!
if two strangers happen to be smokers, they break the ice faster over cigarettes.
i think you start to get addicted out of habit- when i first started, i'd smoke maybe once a week or so when i met my smoker-friends and never feel cravings or urges.
once you start doing it regularly and the habit forms, eg "every morning when i wake up" or "every night before i go to sleep" or "after meals" etc, it gets much harder to quit.
if you've been smoking for an extended period of time, you get chills and fevers from lack of nicotine when you don't smoke.
but at the end of the day i think it's mostly the habit that's the hardest to kick. there is a sense of familiarity and security having something between the fingers, and the deep breaths and puffs are quite satisfying.
i've never really been in it for the so-called "stress-relief" though. I enjoy my cigarettes.
But please don't start just for the sake of it, it's a very expensive habit here in Singapore, it's unhealthy, and it smells bad.