Why are some substances like nicotine difficult for users to stop using?

Because nicotine contains addictive chemicals.

Explanation:

Because it is addictive. It usually only takes 1 cigarette to become addicted, because of the ingredient, nicotine. Nicotine releases dopamine, the same "feel good" hormone that exercising does. (The better option would be to just exercise to feel good its healthy anyways and it's the healthier option.) Nicotine has been reportedly claimed to be as addictive as heroin or cocaine ( being in love is supposed to give that same high feeling like cocaine does. It also releases dopamine. Being in love is the better choice lol.) Once you start to get addicted it's hard to stop.

the answer is d

hope this and you do good