How does nicotine produce a feeling of pleasure

That nicotine calms the person, makes them feel relax, and happy.
Nicotine also activates areas of the brain that are involved in producing feelings of pleasure and reward.  nicotine raises the levels of a neurotransmitter called dopamine in the parts of the brain that produce feelings of pleasure and reward. Dopamine, which is sometimes called the pleasure molecule, is the same neurotransmitter that is involved in addictions to other drugs such as cocaine and heroin. Researchers now believe that this change in dopamine may play a key role in all addictions. This may help explain why it is so hard for people to stop smoking.