This is a question, to which there is a long answer, supported by a huge set of reasons.
In fact even before getting to the last reason on the list, you might very well find yourself starting to like the decision of throwing away the habit.
Knowledge of how toxic cigarettes are to your body and the positive changes that stem in your life after you've given up cigarettes can and certainly add more muscle to your arsenal of reasons against smoking.
Understanding the Quitting process
The decision to quit smoking is a giant leap towards a better life. However this leap can only be successfully accomplished if you know what you are up against and your physical, mental and psychological state of mind.
If you begin to smoke again, don't be disappointed or feel guilty. Instead, try to understand what the quitting process is like. In fact, it might take several attempts before you quit for good. Only when you know about smoking, how nicotine affects smokers and how hard it is to stop, you can meet this challenge with ease.