Why does smoking one cigarette lead to a relapse?

I have not quit yet but I am preparing.

Does the relapse happen for the same reason you get addicted in the first place thinking you can have one more and quit whenever next thing you know you smoke a pack a day?

Or is it more like the addiction is always there waiting to come back and smoking that one flips the switch and you have to start at square one with withdrawals and cravings again?