Does nicotine modifies your brain cells so you will always crave it?

Oh, ok. I am glad I never started. :)

What tobacco companies have done is selective breeding of tobacco to manipulate the nicotine levels of tobacco to addict smokers. Some critics argue that nicotine is a drug and that this proves that cigarettes are a nicotine delivery system and that they should be regulated as a medical device. Our brains use a number of neurotransmitters to influence how we feel. Dopamine is the neurotransmitter released when we feel satisfied by something whether it is a thanksgiving dinner or pride over a job well done. Nicotine mainly causes a release of dopamine which in turn makes us feel good. Addiction occurs when our brain gets accustomed to the increased levels of dopamine nicotine produces which becomes our new normal. When the dopamine falls back to natural levels we feel bad and seek the satisfaction we are accustomed to getting from nicotine.

So is this alteration physical in nature? What if the person stays clean, will the normal go back to what it used to be?

Addiction causes both physical and psychological changes to the brain (which in fact are connected). An addicted smoker who quits smoking will adjust to normal levels of dopamine. They will still have a mental association of feeling good and satisfied by smoking so even though they are over their addiction they still have the habit of smoking to raise dopamine levels. Not sure of the brain ever fully adapts back to normal levels. Even years after quitting, opiate addicts still have very low pain thresholds. The endorphins their brain naturally produces to control pain are no longer effective since their brain became used to the much stronger effect of opiates.