The birth of my first child.
I've been an on-again-off-again smoker for about 7 years, averaging about a pack a day. I got engaged 6 months ago, and my fiance has never had an issue with me smoking. Her only rule is that, when we have kids, I never smoke around them. I explained to her that I had planned to quit before our wedding.
If that fails for some reason, I know that the moment my fiance gets pregnant with our first I will quit, no matter what it takes.