I usually pick them up to cope with intense emotions. Not healthy, I know. After a few weeks of smoking 2 packs per week (not per day, thankfully), I begin to feel intense anxiety from smoking. The longer I continue the habit, the worse the anxiety gets. It escalates to the point where thinking of a cigarette makes me sweaty and nauseous, and actually smoking one will trigger a panic attack. After a few of those attacks I can drop the cigs with minimal fuss.
Anxiety feels like a curse, but in this case it acts as a restoration force against my tobacco addiction. Life is strange.
(No, anxiety is not a good thing, and no, you should not smoke cigarettes)