What are the theories of substance abuse?

If you mean the mechanism, substance abuse involves a path of neurons in the brain. In simple terms it is known as the reward pathway and scientifically known as the NAC ( nucleus accumbens ) pathway or " positive reinforcement" pathway.Activation of this circuit of neurons will cause positive reinforcement. (Positive reinforcement is when a stimulus will increase a particular behavior e.g winning money in a jackpot will make a person play it again repeatedly ). The abused substance is responsible for activating this circuitry, hence repeated behavior i.e taking the substance again and again.

Note: The NAC pathway is normally activated by a variety of stimuli as well . Positive reinforcement is also normally responsible for producing adaptive behavior. ( Love is a good example ).