This morning I thought of addcition as a power struggle, that is, a battle of motives.  In each case motives in the psyche struggle to achieve priority.  Each motive has a desire value that is increased or decreased according to learned knowledge, abstract perception, and sensational experience.  When the ratio is tipped in favor of the motive then it gains power to drive behavior.  This is counter balanced by the inhibition of the opposite or cautious motive.  In the case of addiction, the euphoria motive suddenly gains so much desire value that the ratio collapses completely and drives the behavior blindly.