Why is it difficult for a person with an addiction to stop using a drug?

because its addictive, that's why the consumer feels like they need more

It depends what you mean. if you mean drugs, there are stuff like nicateen in them and crap to make you addicted. if you mean Video Games, (maplestory often) its because you feel you do not like your real life, or want to escape it. if you mean sexual addiction, such as watching porn on internet, it's because hormones in your body begin to crave that stuff, which is inapropriate. overall, addictions, have something to do with your body/life