The difference between extroverts and introverts comes down to a person's sensitivity to the neurotransmitter dopamine in the brain. When we encounter "rewards", dopamine activates positive emotions, such as, euphoria and elation, and motivational feelings like desire and craving.
Richard Depue and Yu Fu from Cornell University, USA, here explain why extroverts consistently seek out rewarding environments: in extroverts, dopamine has a stronger capacity to promote the formation of mental associations between environmental stimuli and rewards.