A few to say the least:
1) Important one: My Dad was diagnosed with COPD a couple of years ago. Primary cause was coal dust, aggravated by smoking - he quit almost twenty years ago.
I also saw at a young age my granddad die from complications of COPD, a smoker until his final hour. Watching him take the oxygen mask off to light up, is enough to make anyone re-think their reasons for continuing.
2) Health - Although a bit overweight I've noticed recently moving some heavy kit around at work leaves me sweating and out of breath a lot more than it used to, and I've lost a few stone recently! Considering I'm only 25 and watching a marked deterioration of my lung capacity is never a good thing
3) Boredom - I can go 10 hours at work with only occasion minor cravings, but at home during an advert break, or just when my partner is busy upstairs I'll go for a smoke. I didn't really enjoy these, they just filled the time. Why kill yourself just to kill a few minutes?
4) Marriage - I'm getting married in May, I do not want to enter the next phase of my life as a smoker. I intentionally booked in to the same hotel as my parents (as far as they are concerned I quit back in 2002) to back myself into a corner to quit now. No one wants to be going through withdrawal during their wedding!
5) The stench - Do we smell! After the pre-bed smoke, I'm always told how much I stink out the bedroom, not the nicest thing to hear!
6) The hypocrisy - I work in a school (non-teaching), how can you tell the students off for having a crafty fag behind the bike sheds, if you've just come back from one yourself?!