What causes people to be addicted to television, drugs, smoking, chocolate, alcohol, etc...?

When people do someting that they think is interesting for them. It will become a hobit. In addition, they will do this thing everyday. When they be addicted by this thing, they can't stop to do it. For example, when people begin smoking, it will become a hobit. They will just smoke more than before. If you want to quit, it is difficult to do it. They need have a strong spirit. If they don't have a strong spirit, they will just keep to smoke gor a long time. Moreover, I like to eat chocolate. When I begin to ear chocolate, I won'y stop because the taste is perfact. Therefore, I will eat one and one more and keep to continue. Finally, I will get a lot of weight. As a result, if people be addicted by something, it is really difficult to control themself.