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Because our brains want instant gratification.
Yes we know it is bad, we know the great benefits of Not Smoking, we know all this but our brain wants gratification NOW.
That is why when I first started, I used delay tactics to let my cravings pass.
I kept smoking for many reasons. One of the reasons was because secretly, deep inside I think I'm special, that all the bad things smoking does to the human body, it wasn't going to do that to me, it wasn't going to kill me. And then two different doctors convinced me that I'm not special, that all the bad things smoking does to everyone else, sooner or later, it was doing to me and, in my case, sooner rather than later, starting with high blood pressure.
I can't do anything about the thousands of cigarettes I've already smoked but I can do something about the next one. Choosing not to smoke, now that does make me special I think!