It's not just a habit. It's a drug addiction, and the actions of a 'habit' are borne out of that addiction.
Once you understand the drug addiction side of it, you begin to understand that it is the addiction which fuels your behaviour.
You're addicted to a drug, so you convince yourself you enjoy it, just like an alcoholic 'enjoys a drink'. They dont take a sip and go "oof thats disgusting...lemme have another".
The addiction conditions you to keep getting that drug into your body.
It's no coincidence that cigarettes are in packs of 20.
When you have a cigarette, you 'top up' your nicotine levels, you feed that drug addiction, kinda like the heroin addict who drops the needle in a state of bliss.
Half an hour later, the nicotine levels in your body have dropped, so you think you "want another cig"....your body is simply crying out for a topup of nicotine, so you reach for another cig.
Say you get up at 8 am and go to bed at 10-11, give or take work times, and other things...that's one fag roughly every half hour....ie 20 a day
All habits we have created ourselves, theyre not behaviour natural to all humans.
When I go to bed at night and pass the front door, it's habit that I check the door is locked. That's a habit...a habit I started myself. I checked the door each night for a few days...and after so many times it becomes 'habit'....'second nature'...just like smoking does....it's an unconscious decision.....but it was put there by the conscious mind. So how do you reverse that? Simple....Make all your decisions consciously. When you think you want a fag, consciously say to yourself "No I don't want a cigarette now".
That's how you break any 'habit', override it with an opposing conscious thought.
That, coupled with the addiction means its so easy to get hooked on smoking, but once you understand it, it becomes much much easier to break the vicious cycle.