Good topic OP!

MootWorm, yikes.

My family smokes... my parents and brother, and several relatives as well. I personally vowed to myself (apart from never starting to smoke - because I just couldn't nor do I want to) that I wouldn't be with someone who smokes. I don't mean this in any "oh I'm better than you blah blah blah" way... I simply couldn't handle the possibility of losing someone close to me, by a means that was definitely avoidable.
Like other people, I also get headaches around cigarette smoke. But the biggest thing for me would be losing someone important in my life... it has already happened, sadly, by a combination of habbits - including cigarettes.

I have asked my family members before why they started smoking. I could tell you that my brother started because of the crowd he hung with. He started working as a bouncer at our only town's bar at the age of 18, although he had smoked before then. But that is where he upped the anti, and began smoking regularly; as he puts it, everyone else smoked on their breaks and he wanted to be a part of it.

My parents both started in their late teens as well. They are in their 50's now, but I think they don't really have reasons... apart from "it was just the thing to do".