I know smoking is going to kill me, I've no illusions about that.
On the other hand I could be killed tomorrow driving or walking down the street. I could be shot dead because someone doesn't like the cut of my jib. I could die in a house fire, I could slip and break my neck, I die any number of ways.
I enjoy smoking. I enjoy the taste of it, I enjoy the act of it and I enjoy how it settles my nerves when I'm on edge (which is funny considering that nicotine is a stimulant). So why stop doing something I enjoy because it'll kill me some day? If I were to do that then I should also quit drinking soda and eating candy (diabetes runs strong in my family), I should stop eating red meat, eggs, butter and any number of things that can lead to heart disease.
As far back as I can remember I've been of the opinion that if I have a choice between enjoying a short life or living a long, boring and miserable life I'll choose the short enjoyable one.