Depression in my case. I started smoking before high school as something to fill the void, only realizing much later that I've been severely depressed for a long time. (SSRIs are an amazing tool in my case FWIW)
Once you get used to it, it's an amazing social tool -- making new friends/acquaintances becomes a piece of cake. I travelled a lot for work and, as a smoker, I would instantly integrate with the other smokers. It's just something fun that leads to pursuing other social activities together. I've seen ADHD mentioned in this thread and that makes sense, the habit as "something to do" is a powerful grip on top of the chemical addiction.
Once you get used to it, it's an amazing social tool -- making new friends/acquaintances becomes a piece of cake. I travelled a lot for work and, as a smoker, I would instantly integrate with the other smokers. It's just something fun that leads to pursuing other social activities together. I've seen ADHD mentioned in this thread and that makes sense, the habit as "something to do" is a powerful grip on top of the chemical addiction.