...the Reticular Activating System (RAS) ...responds to stimuli like food, or social contact. For example, it controls the amount of saliva you produce in response to food. A good food stimulus is lemon juice. Squeezing lemon juice on to your tongue makes your mouth water, and it does this because your RAS is responding to the lemon juice.The fact that there is a physiological difference in response between introverts and extroverts! So, no, we can't just "get over" our aversion to too much social stimulation.
Scientists now think introverts have increased activity in their RAS and therefore increased production of saliva. The theory is that the RAS in introverts has a high level of activity, even when it isn’t being stimulated. So it only needs a small stimulus to produce a large response. This means that introverts are likely to produce a large amount of saliva in response to lemon juice. But because the RAS also reacts to social contact, introverts react more strongly to meeting people too.
In extraverts, on the other hand, there is a low level of activity in the RAS when it isn’t stimulated, so they require a much larger stimulus to generate a response. So they usually produce less saliva in response to lemon juice than introverts, but are more comfortable with social contact. ~ BBC, Lemon juice experiment
12/4/07: I thought that these articles on Differential Psychology (with studies related to extroversion, intelligence, and neural efficiency) were a good follow-up.