Why so many smokers relapse after having been quit for many months

This is an except from my book, "How to Win at Quitting Smoking": 

If you wanted to drop weight, you need to be willing to change your food intake and increase your physical activity. This takes a commitment, making a Plan and taking action. Slowly over time the pounds come off.

If after six months you went to a birthday party where your favorite chocolate fudge cake was being served and this