Because its addictive! It effects your willpower, for some people they can't even think about something without lighting a cigarette
Most smoker will light a cigarette if they have some tension or pressure and this makes it hard to quit smoking. Lastly its even hard if you have friends who smoke
In gathering when your friends light their cigarette its very tempting for person who is trying to quit on it
Normally in such situation a smoker's response is "
Aj In Kanjron K Sath Pee leeta hon Kal say Pakay choor don gaa... "