What are the causes of a substance to be addictive?

Simply because ..: There is a "pleasure center" in your brain designed to respond to good food, good relations, and good work. A substance or behavior that generates a more than expected reward can lead to incentive sensitization if the person is biologically predisposed to addiction. The faster the drug effect reaches and then leaves the brain, the more craving will develop, such as with nicotine or crack cocaine.

Pleasureable: If it makes you feel good (or at least different) then it has the potential to become addictive.