Why do they smoke more when...?

My parents have been smoking since they were young, probabily since they were teenagers. I know back then people didnt know they were that bad for you so they got hooked on them. They are still addicted to them and my question is why do they smoke more when they get annoyed or stressed. I'm not stupid, I completely understand that the nicotine in them are addictive and thats why they won't quit and regardless of their mood, they smoke but when they get mad, annoyed, or stressed they seem to do it more. Why is that? Do cigarets just relax your body or something? I don't get it.