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.
Over four consecutive days, Depue and Yu gave methylphenidate, a drug that activates dopamine, to volunteers to increase their perception of subjective reward. They found that extroverts more readily associated stimuli with reward, as shown by an increase of those motor, affective, and cognitive processes that are regulated by dopamine.
These results suggest that extraversion is associated with individual variation in the capacity to encode rewarding stimuli in memory.
URL: http://www.frontiersin.org/Human_Neuroscience/10.3389/fnhum.2013.00288/abstract