It is quite simple: someone is offered a cigarette, and they accept. And they have some more, and it is pleasant and social and/or stress relieving (both in ritual and physically: warm smoke in the mouth and throat is pleasant, as is the slight buzz, as is the act). This will often happen at a young age (teens, early twenties), within social settings. And then the addiction and ritual are the thing, and that thing is extremely satisfying. If you do not and have never smoked, then this will seem stupid, and it is, that would be a completely fair assessment: it is unlikely that any smoker will recommend that you become a smoker. Note that many people who give up smoking will replace it with an equally pointless (albeit generally much, much healthier) addiction/crutch that gives a mild buzz and requires some ritual habits (running, for example).