i don't believe you can put it down to one thing. as pat points out, and we all have examples in our families, there are plenty of heavy smokers who have long lives. smoking seems like too easy of an out for the docs. also its a bit dangerous as it kind of blames the patient and, in a health system like ours here in uk, could lead to prioritising patients who have just made different lifestyle choices. I mean, no one asks you how many big macs you eat when you present with BC or how much you drink or what fruit and veg you get through in a week. most likely its multiple causes; bad nutrition, location, job history and a genetic predisposition etc etc etc.
the best reasons to give up smoking are that your life just feels a lot better. food is more enjoyable, you can exercise without getting out of breath, your skin and eyes look better within days of giving up and you save a shedload of money. these are real, tangible, immediately noticeable benefits.
t