Happens the same way people become addicted to anything else. You try it one, enjoy the high, and decide to keep doing it. If you're unlucky, you'll get addicted. And let me set this straight right now, you absolutely get a "high" from cigarettes that most people refer to as a buzz. The problem is that the buzz has diminishing returns and nearly all people that are actually addicted to nicotine no longer get this feeling...they have to smoke just to feel normal.
Like basically every other major drug, I decided to try smoking out when I was 18, mostly because I'm a cocky bastard and I feel that I'm "above" addiction (and it turns out I am). It's a great feeling and I can understand why a lot of people smoke, it just sucks that so many people aren't able to handle it in moderation. If you're only having a couple a week or so, you're not going to smell terrible all the time and I'm sure there wouldn't be some ridiculously high chance of adverse health problems either...but the risk of physical addiction just seems too high.
--
http://soundcloud.com/thundersheep/lunatic-asylum
http://soundcloud.com/thundersheep/summer-rain