My own theory on the subject is that introverts have certain social expectations that they feel that they must fulfill, hence it's very energy draining for them to socialize. It makes them need to try to be able to socialize. They try to find the right words to say rather than just speaking their mind, and that gets very fatiguing. Extraverts, on the other hand, feel comfortable with being themselves, hence they are often known to speak before they think, as opposed to introverts who think before they speak. Also, if you think about it, all of us have had times where when we socialize where we feel energized, and we've all of us have had times where when we socialize, we feel drained. If you think about when we feel energized, isn't it when we share something that energizes us, something that gets us excited, something we feel passionate about. So what really is it that makes someone an introvert or extravert if everyone has moments of introversion and extraversion? While when Carl Jung originally popularized the concepts of introversion and extraversion, he made the traits seem like a type, you're either one type or the other, while in personality research today, it's much more popular to regard it in a continuum.
There are also many interesting studies on the link between extraversion and dopamine, particularly in the brain's reward systems. It is found that extraverts have higher levels of dopamine in these reward centers. This has two main effects, first is a higher positive affect, or in other words, they are happier, and secondly, higher dopamine levels also mean that that are much less inhibited in acting out impulses. It is found that in order to execute a behavior, a certain dopamine level must be reached for this to occur. This seems to parallel why introverts tend to overthink in social situations and why extraverts just speak their mind.
In this sense, I feel that introversion is almost a form of pathology. It is very rare to almost non existent for one that is extraverted to be depressed, but depression is highly correlated with introversion. But at the same time, I feel that there are many benefits to introversion. The facts are, it's nearly impossible for anyone to be perfect content with the way they are at all given moments of their lives. A leading psychiatrist says that depression is not a human defect at all, but a defense mechanism that in its mild and moderate forms can force a healthy reassessment of personal circumstances.
Given this, I feel that introversion and extraversion are less of personality traits, but more as personality states of a person.