It depends on what they are addicted to.
Most drugs are addictive in and of themselves; other substances, like food, become psychological crutches.
How or
Why people become addicted? Well, those reasons are as diverse as the population. Some initially try a drug to "fit in" with a certain crowd of people. Some to "
help" ease the pain of some physical trauma or
emotional event. Some out of curiosity. Some from rebellion. Some for the high or the risk.
We, as an American Society, are pushed from all sides to take drugs to "fix" a myriad of problems -- the ads are all over TV, our friends are all on Prozac, our kids are all on Ritalin, the anger management crowd is all on Zoloft, grandma is on hormone replacement therapy, someone's sister is taking fertility drugs, someone's brother is taking some weight-loss miracle pill, grandpa is taking Viagra or Cialis, and the list goes on and on. Meanwhile, new drugs are emerging all the time to "fix" whatever ails you.
While some people say that it's only the "weak-willed" or already genetically predisposed folks who become addicted, I think it can happen to anyone who willingly indulges in mind-altering or physically addictive drugs.