On the opposite emotional spectrum from stress is boredom. David Sedaris has a hilarious short essay about how smoking helped him with his OCD and tics in college because it gave him something else to do [source: A Plague of Tics]. Sedaris is not alone -- there is a startling correlation between many mental disorders and smoking, particularly schizophrenia, due to a lack of dopamine in the brain that can make nicotine intake a more powerful or enjoyable experience [source: Schizophrenia.com]. A lack of your brain's "feel good" chemicals is also often lacking when you feel bored or run-down.
Many smokers talk about how it's great having something to do with their hands. There are lighters to fiddle with, ashtrays to move around, packs of smokes to pack, unwrap and store. The cigarette itself allows the hand and mouth to be physically active even if the mind is at rest. Some people bite their nails or twirl their hair when bored: smokers smoke. Being bored in this way is a tough trigger to tackle because boredom, like stress, is not always avoidable.