I've had a few that all added up to "I have to quit NOW."
My first was when my 3 year old son looked at me as I was on my way out to the garage and said, "Are you going to smoke AGAIN, mommy?"
Then, I received my first child support check from his father, and immediately went out and bought a bunch of cigs with it.
Finally, my boyfriend (who is extremely asthmatic, compared to my mild seasonal asthma) had a severe asthma attack after spending a day in the car with me, and we had to leave a movie we were seeing to rush home so that he could use his nebulizer, running multiple red lights and making several illegal turns along the way, because he actually COULD NOT BREATHE. And I thought, "Wow, if I can get my nic fix without affecting his lungs, I'm a real b!&$# for choosing the harmful alternative."
I still slip up and have the occasional analog, but never at home, never around him, and never around my son.