I do no believe most of the people know why they are having the habits that they do..it is just a habit and there are likely to be many reasons for it. For me the reasons were:
1. I was paying 10$ for a pack of smokes - I smoked 1 pack a day
2. I was married and my wife hated it when I smoked inside the house. Once my son was born, I was told I am not allowed to smoke inside the house at all!!!
3. I had to smoke outside on the veranda which was not covered. That means that when I wanted to smoke and it was raining, I really had no choice....it was either not to smoke or smoke under the rain..I would often choose to smoke under the rain Very unpleasant.
4. Could not smoke in my office. Had to go 5 floors down to a designated area...very annoying and time consuming
5. Some minor concerns about the long term effects of smoking cigarettes.
In short it was the combined forces of money, time, convenience and healthy concerns for those around me and to some extent myself.