Why loot systems are so addictive

From Diablo to Path of Exile, from Destiny to The Division, some games are highly addictive because of their tight controls, meticulously crafted loot systems and highly sensory audio design. Design decisions alter brain chemistry. The random loot drops create unpredictability that builds anticipation and excitement. Chasing new loot gives direction, and obtaining it results in an immense dopamine rush.