I think the main reason it helps people reduce stress is they have to leave a situation to go outside or to another room in order to smoke and they’re taking a deep breath. I found the feeling of breathing in the smoke (throat hit I believe it’s called) and then breathing it out and seeing the smoke cloud very satisfying. Similar to chewing gum I suppose. Also, I rolled my own filter-less cigs so I had the added element of rolling it and picking out strands of tobacco hanging out of the end whilst smoking as a kind of fidget action. I prefer vaping now and don’t think I’d go back to smoking cigarettes regularly but I have started smoking a cigar with a beer some weekends just in the last few months. As some have stated the actual feeling from the nicotine is similar to caffeine but more short lived.