'cause your body gets used to the nicotine

Because nicotine is physically addictive & the act of smoking is also psychologically addictive so it's a double whammy

I agree with@elabeth

totally agree with elabeth, I had a hell of a time quitting! 3 years now and I still crave them from time to time

i've been quit for 15 years and i still crave em once in a while

but the longer you go without smoking, the shorter the cravings are when they come

any habit takes at least 30 days to break and works better if something is substituted for it. That's why some eat or chew gum.